Concrete Bat Box

Igloo 303

On my last visit to the Hop Brook conservation land, I had noticed that there were signs along the abandoned railroad tracks just to the west indicating that the property to the north belonged to the Assabet River National Wildlife Refuge. Later I searched out what I could find about it, to see if it was a suitable place for additional hiking. That resulted in a long post for today. I found the history of this place fascinating, and wanted to write up what I’d discovered.

Not only was it suitable, it turned out to have some very interesting history, as well as structures I’d not expect to find in a wildlife refuge: giant concrete bunkers. This I had to see, so last weekend I went for a visit. That just whetted my appetite, and I was on vacation, so I went back again mid-week. It was hot (~85°F, or 30°C) and very muggy, but I barely noticed that. The hiking was very good, with few other people, and long stretches of walking through dim forests of huge trees along a firm dirt path. In other places there were paved, graveled, or rutted dirt roads. I saw a tree half cut through by a beaver. And through much of it were scattered the “igloos”, as the Army called them.

An “igloo” is a former U.S. Army ammunition storage bunker, of concrete, steel and dirt. The Army nickname probably comes from the mounded shape and the single small entrance. When built, earth was piled over the concrete, in part to conceal it and to help insulate the contents from the sun. As a result, today they disappear into the landscape, with fairly large trees growing atop them. Originally built to store ammunition, today they store…bats. Or at least, that’s the plan.

But let’s back up a bit: on the eve of the Second World War, the United States had given away much of our reserve of ammunition under the Lend-Lease program, and production capacity was low. Both for Lend-Lease purposes and anticipating eventual involvement in the war, billions were spent to create civilian munitions factories. By 1942 those factories were producing “ordnance” (weapons and ammunition) at a phenomenal pace, with approximately $106 billion worth produced during the war in total; a massive increase in production.

Existing depot space wasn’t designed for the volume needed. And while weapons could be stored in an ordinary civilian warehouse, although usually only on the ground floor due to weight, explosives required more care. To distribute the output from this new set of factories, a vast network of storage depots was created.

Factories were intentionally located more than 200 miles (322 km) from either coast, for security against bombing. There were depots near the factories to store production before it could be shipped, large inland depots for long-term reserve storage, and in some cases depots at seaports where munitions would be loaded onto ships for transport. In others, loading would be straight from train to ship.

But trains full of explosives can’t just be left parked until needed, and coastal depots were at risk from enemy ships, at least in the early days of the war.  In some places there were large depots relatively near ports and trains could run as needed. But not all. So the Army’s answer was to build “back-up depots” for ammunition just far enough inland to be safe from the heavy guns of an enemy warship, and in a location that was both secure against sabotage (a big concern at the time) and far enough from houses or industries in case of accident.  Few of these were actually built (I found reference to two, and they’re usually omitted entirely from lists of depots).

The Maynard Ammunition Backup Storage Point (or Backup Ammunition Storage Point, I’ve seen it written both ways) was one such depot. That was the original name, it’s had several since; locals called it the “Ammo Dump”. It served the port of Boston, Mass. from a location 24 miles (38 km) from the coast, adjacent to two railroad lines (both owned by the Boston & Maine Railroad), in a mostly marshy area on the edges of Hudson, Maynard, Stow and Sudbury bounded on two sides by waterways and previously occupied by a few farms. It was an ideal location.

It’s not clear just what it’s role was, really.  I’ve found references that all Liberty ships (the primary logistics ships carrying supplies to Europe) were loaded at two facilities, the nearest in New York.  The Navy had its own facilities for mines, torpedoes, and shipboard ammunition (more on that in a bit).  It seems likely that this was used for ammunition being sent on troop ships, as Boston was a Port of Embarkation for troops going overseas.  This seems to be confirmed by the following comment in a 1994 Army report on the evaluation of hazardous materials at the site (PDF).

“The Annex [a later name for it] was specifically tied to Castle Island Port, the loading point for ammunition being transported overseas through the Boston Port of Embarkation system. When ships were not available for loading or a surplus of ammunition had been received, ordnance would be stored at the Maynard Ammunition Backup Storage Point.” (pg. 3-2)

Here fifty storage bunkers rated for high explosives were constructed (I’ve also seen the number given as 30, but multiple sources confirm 50), along with rail lines and supporting buildings. The bunkers were built to a standard design, 12′ (3.7m) high and 81′ (25m) long: the steel-reinforced concrete structure was a long half-cylinder, on the outside the sides and closed end were piled with dirt tapered to the ground, making the top much thinner and weaker.  This was to direct the force of any explosion upward, to avoid affecting nearby bunkers.

At the front of each bunker, the only part not covered with earth, was a small loading dock and the single steel door. Railroad tracks ran past the bunkers which were located about 400′ (120m) apart, so boxcars could be positioned at the dock (presumably one at a time, for safety). The bunkers could also be accessed by truck, in the event of a problem with the railroad line. The dirt ramp seen here wasn’t an original feature; it was likely added after they stopped using the railroad.

This size was the largest of three standard igloo sizes (the others were 40′ and 60′ long), a design that had been standardized in the 1930’s for both Army and Navy, after lightning caused a chain-reaction string of explosions and fires in 1926 that destroyed almost the entire Lake Denmark depot in New Jersey. The new bunkers were also equipped with a complex set of lightning rods, although no trace of those remains. Although other kinds of munitions storage facilities existed, the igloo was the most common one built during the war (over 5,000 were constructed). A similar design was used after the war, although later ones had square walls and a tapered roof, to increase capacity.

Construction of the depot began in 1942, when the Army took 2,750 acres (11 square km) of land in Maynard, Stow and Sudbury (plus a sliver of Hudson) by eminent domain, giving the occupants just ten days to leave and paying them a nominal sum (which the residents felt was a small fraction of the true value). A small railroad yard was built adjacent to the B&M-owned Central Massachusetts Railroad line on the south side, well away from the bunkers, and a network of rail lines and roads built throughout the depot.  There was actually land to build even more bunkers: the northwest corner was never occupied by the depot (per a 1950 topographic map in a DOT report, see this PDF).

Although there was a second railroad line on the north side, the depot wasn’t connected to it, likely because those tracks were abandoned in 1943, and probably were out of use well before then (track abandonment dates come from Lost Railroads of New England, 3rd Ed, by Ronald Dale Karr). From the EPA reports below it appears that the railroad yard was closed by 1957 (quite possibly in 1950 with the cessation of use as an ammunition depot). However, there is reference in the Army report to Fort Devens using the tracks to access bunkers for storage in the 1960’s (perhaps only operating within the northern section). The adjacent Central Massachusetts Railroad tracks were formally abandoned in 1980, but operations on that segment had ceased in 1965 when the MBTA took over commuter passenger service (there had apparently been no freight there for some time prior to that).

With just 50 bunkers the Maynard Depot was actually fairly small. Large inland ones had 800 or more. The Navy had built its own depot south of Boston, in Cohasset, the Hingham Naval Ammunition Depot Annex, with over 80 bunkers. Hingham was the primary (perhaps only) depot for restocking ships of the North Atlantic fleet. Apparently battleships didn’t worry the Navy as much as they did the Army, as the Cohasset depot was four miles from the coast. The two facilities appear to have been unrelated; Maynard does not appear on a list of Navy duty stations during the war.

After the war the Maynard depot was no longer required and closed as a depot in 1950, but the government is not quick to let go of military bases of any sort, particularly with the Cold War heating up. In the 1950’s the land was used, among other things, to test explosives and other munitions (per the above Army report and a pair of EPA reports, see here for one, and tables and graphics for the original report here). After that, it was transferred to the Army’s Natick Labs in 1957, which develop equipment for use by soldiers. In their hands it was used to test air-drop systems for delivering equipment, as well as burn-testing, pesticides and other functions. It was also used to store and dispose of chemicals.  In 1982 it was transferred to the control of nearby Fort Devens (as the Sudbury Training Annex, its final government name and the one used in most reports) and used as a training ground for the Army, as well as both Federal and State agencies (it also continued to be used for storage, disposal and other purposes). In 1995, with the Cold War over, it was one of many facilities identified for closure due to spending cutbacks. But it needed to be cleaned up first, as it was an EPA “Superfund” site due to the toxic chemicals dumped and buried there over the decades.

In 2000 most of it was transferred to the Department of the Interior’s Fish and Wildlife Service and incorporated into their nationwide complex of wildlife refuges, and in 2005 it opened to the public as the Assabet River National Wildlife Refuge. About one-third of the northern section, ~500 acres on the northwest corner, was transferred to the Air Force, which operated a weather radar installation on a hilltop there (it’s apparently since been closed). There are ongoing discussions regarding Fish & Wildlife gaining the remaining portion to enlarge the refuge. In 2010 a Visitor’s Center was opened, with a set of displays on the wildlife and a small gift shop (I picked up a couple of books and the usual refrigerator magnet on my visit last weekend).

And it’s a very good wildlife refuge. Although it was in use for the last 72 years, and some uses were quite inimical to animals, much of that was confined to a few areas, and the forest and wetlands seem to have been mostly left untouched.  There’s a mature forest of mixed pine and hardwood covering much of the land, with some oaks two feet (60+ cm) across at the base. The result is a wilderness close to two miles across, with a large wildlife population, located in the heart of Boston’s suburbs. A pond and small waterway cater to migrating birds, and wetlands support many other species. There is an active project to reintroduce a species of endangered turtle. The former human uses are visible in their traces: aside from the bunkers and the earth fills where roads and rail lines once ran, there are foundations from old farms and military buildings, and stone walls from colonial times running through the woods.

Humans are allowed in too, although that’s limited to the fifteen miles of marked trails. Major trails allow both pedestrians and bicycles, and secondary trails (many of them former railroad lines) allow pedestrians. Dogs and motorized vehicles are not allowed. But it’s lightly-used, in part because it’s not well-known. Most maps haven’t yet been updated. My car’s GPS still shows it as a blank gray space marked “military reservation” with no roads, although my hiking GPS shows an old topographic map with roads and long-gone rail lines marked. There’s a trail map available from kiosks at the parking lots, and at the visitor’s center, and it can be downloaded from the main Refuge site linked above. The gates seem to close on the same schedule as the Visitor’s center (i.e., early), but the reservation itself is open until sunset and there’s a parking lot outside the southern entrance.

I was curious about the extent and layout of the old rail lines and to what extent the bunkers had survived. The DOT report I linked above has a number of maps, including a 1950 topographic map showing the rail lines at that time, as well as small squares for bunkers and other buildings. I took that, and GPS readings from my phone’s camera, and figured out how the modern trails relate to the original rail lines and access roads (well, the ones that were open; I had to trust the map for the others). But there were a couple of places that didn’t seem to match. The Army report noted above, however, had more detailed blueprint-style maps of the cleanup sites, and clearly showed the bunker sites, right down to the bunker numbers on one map. This map was in line with my experience. From my walks I was able to spot 25 of the bunkers, and no space that should have held one per the better map did not, so it would appear that all were left intact (although a few have substantial trees in front, and may not have been used since 1950).

I didn’t explore all the roads mapped in black though; that’s for another hike. And many of the former rail lines are closed, and those bunkers are mostly invisible from adjacent lines. I’ve summarized my findings in a map. Unlike my photos, the map below is released to the Public Domain, use as you wish (since the topographic map is a US Government publication, that part is already public domain, at least within the US, by definition), but even elsewhere its use here probably falls under Fair Use provisions, as transformative, partial and non-commercial).


It may be best to visit in the spring and fall: due to the marshy land there are a lot of bugs, and even bathed in insect repellant I was bothered by flying insects bouncing off my face any time I stopped moving, and I received at least one mosquito bite each trip. I’ve also read that ticks are a common problem, although I didn’t pick up any on my visits.

Before it was opened to the public, several dumpsites and spills had to be cleaned up (and one was largely excavated and then capped, and continues to be monitored), old well shafts filled, razor wire removed, and all of the rail lines taken out. Much of that was done by volunteers. All wooden structures were also demolished. But the bunkers are too solid to easily remove, so they were merely cleaned out and padlocked. Tours of one are given a couple of times a year by an associated non-profit organization.

But what do you do with fifty artificial caves on land restricted to wildlife and light recreation uses? Several are currently being monitored to determine if conditions inside are suitable for bats to nest.  Much of the land is boggy wetland, and as I mentioned the insect population is very high. Bats would be likely to find the area quite suitable, and would undoubtedly help keep the insects in check.

I like this photo because it shows how nature has moved in on one of the bunkers, with substantial trees growing out of the earth atop them. This is in fact one of the more visible ones, the first you come across up the trail from the Visitor’s Center, identified as number 303. Typically they are all but invisible, noticeable only if you are within a hundred feet (30m) or so. Several times on my walks I was surprised when one loomed out of nowhere behind a screen of trees. It’s amazing how quickly nature has taken back the land. The photo also shows the old access road turned hiking trail, illustrating how the land has been reused. The fact that the bunkers may find a new use as a home for bats is just icing on the cake as far as reuse.

This image is an HDR, taken from the usual bracket of five photos at 1 EV offsets, this one centered at 1/160th-second exposure, with f/11 and ISO 800, and with the lens at 24mm. My circular polarizing filter was not used.  Processing took one of the presets (“detailed” I think) with only minor tweaks to boost contrast and reduce exposure. The usual Edge Sharpening was done.


One thought on “Concrete Bat Box

  1. I grew up on Willis Lake. I was born in 1958 and lived there until age 18. Growing up I swam, ice skated, blueberry picked in the lake and hiked the woods in this area. Two years ago I found out I have three cancers, my sister and many of friends from that neighborhood also have multiple cancers. I have done some research with people who grew up in Pine Lakes and have found 58 people who have cancer from that area many multiple cancers. If I add in autoimmune diseases I know of over 178 people. I believe if I could find out how to contact more past residents the numbers would be very high. I remember big barrels in the woods and on rainy days the puddles near them would be purple or blue/green. I believe that some of those chemicals stored there leached into the water of Willis Lake, contaminating the water, blueberries and due to well water our vegetable garden’s. I believe they are not telling us everything they should about this land and because of this many now have terminal cancer cells in our bodies. I fear for the people who are not aware and grew up in the area.

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