Midewin National Tallgrass Prairie
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  • 23 Sentinals Of War

    One of the many bunkers stll a part of Midwin's landscape.

    The structures you see to the north of the trail go by several names, bunker, igloo, and magazine. In the United States, buildings for storing ammunition and explosives have been necessary since 1775. In the late 16th century the word magazine referred to a storehouse used to contain arms, ammunition, and explosives needed for military use. By the mid-19th century, the meaning of magazine changed to describe the chamber for holding a supply of cartridges in a firearm. In modern times, magazine has gone back to being a military store for arms, ammunition, and explosives.

    Magazines from 1775 to 1926 were above ground buildings made from stone or brick. After military conflicts, magazines tended to become overloaded with leftover ordnance. In 1926, an electrical storm occurred at the Lake Denmark Ammunition Depot in New Jersey.  There was a lightning strike on magazine number 8. Within five minutes, this magazine exploded and threw embers and debris as far as one mile away. This explosion was the start of a chain reaction causing another magazine and a shell house to explode. Nineteen people were killed and fifty injured. The damage to munitions and structures exceeded $40,000,000. The military searched for the facts in this disaster while the public expressed their horror at this disastrous explosion. 

    The Lake Denmark Disaster spurred Congress to approve the First Deficiency Act in 1928. This Act proposed a joint army-navy board to survey the safety conditions of ammunition storage. Before the Disaster of Lake Denmark, all magazines were above ground structures. This disaster pushed underground magazines to the forefront of safety concerns. The preferred design for magazines became the earth-bermed, concrete igloo magazines. Yes, there are igloos in Illinois today and there have been since 1941. These underground magazines picked up the name “igloo” from their resemblance to the dome shaped Eskimo structures. 

    At the Joliet Army Ammunition Plant, there were 395 igloos in 6 fields. Igloos, on the west side of Route 53, stored trinitrotoluene more commonly known as TNT. Other products produced and stored on the west side included DNT, its accessories, sulfuric acid, nitric acid, and sellite as well as lead azide used for primers, and tetryl used for boosters, fuzes, and detonators. Igloos on the east side of Route 53 housed bombs, shells, and mines. Components of these munitions, detonators boosters, and fuzes, were also stored in these igloos while waiting to be assembled in one of the four Load, Assemble, and Package plants know as LAPs for short. Munitions were then stored in igloos until they were shipped out to the military. 


    Igloos at Midewin are 26 feet wide with lengths of 40, 60, or 80 feet. The floors are poured cement with a 2 foot deep foundation. The barrel arched concrete walls and ceiling are reinforced with steel rods. Blueprints show the cement walls as one foot thick. Three feet of dirt cover the roof with up to fifteen feet of dirt on the back and sides of the structure. The igloo was designed to propel any explosion upward and avoid the chain reaction disaster that occurred at Lake Denmark Ammunition Depot. This design intended for the roof to be the weakest point in the structure. In 1948, there was an explosion in an igloo at an ammunition depot in Savanna, Illinois. The urban myth states that the 1,200 - 1,500 lb. door was blown off its hinges and into the Mississippi River, never to be seen again. The truth, as told by a Savanna Army Depot worker, is that pieces of the door were found but not in the Mississippi River, since the igloos were located some distance from the river. It appears that the door may then be the weakest point of the igloo.

    The arrangement and the distance between igloos was also configured to help minimize an explosive chain reaction like that at Lake Denmark. These igloos are staggered, so no building is directly in front of or behind another. The igloos are 425 feet apart from side to side, somewhat longer than a football field. The rows of igloos are 800 feet apart. The number of bunkers in a group varies. The doors of all the igloos are on the same side to allow easy access by rail. Raw product and finished munitions were moved within the Joliet Army Arsenal Plant by rail. Signs that were once on igloos designated the number of pounds of explosives that could be housed in each building and the maximum number of workers that could be in the igloo at any one time.   

    Any condition that exposes munitions to water can lead to misfires and incomplete detonation and could have a profound effect on their safe use. Igloos sport a cement chimney for ventilation of stored munitions as well as floor drains running along walls to drain any condensation. The bermed earth of the igloo has a second benefit. It insulates the igloo creating a constant temperature range between 40 – 60 degrees Fahrenheit. This is important for TNT storage. A constant cool temperature will mitigate the increased risk of accidental detonation due to extreme high temperatures. Other protective measures taken for igloos included lightning rods to ground the structure, hooking railroad cars to a grounding cable during loading and unloading, and screening of vents to prevent wildlife from entering the igloos and chewing open packing materials. In addition, grass growing around the igloos was kept short to prevent grass fires from getting too close to the explosive contents.

    What you see at Midewin are Army Standard Igloo Magazines or Type 49s. There were a few variations in igloo design but most modifications to the igloo design were made to alleviate the material shortages and slash construction costs occurring during WWII. Over the course of WWII, 10,000 ammunition and explosive storage structures were built across the United States. Igloo magazines provided a safe space for the storage of munitions. This was extremely important in focusing the attention of the military and the public on the military conflict and not disastrous explosions at home.

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