Milner Dam, built in 1905, feeds many miles of canals with Snake River water. The loess and alluvium of the desert provide the soil for the crops, but it’s a thin deposit blown in eons ago. Thirty or forty feet below the surface lies lava rock, a solid barrier to water drainage.
In the 1920s, farmers sometimes found themselves standing hip-deep in mud because their irrigation water had nowhere to go. So, in addition to the miles of canals on the surface, the Twin Falls Land and Water Company determined that they needed a drainage system to take the agricultural runoff back to the Snake River.
Beginning in 1926, tunnel crews dug, blasted, and drilled what would become a tunnel system nearly 22 miles in length. Some 350 men dug the tunnels between 1926 and 1951. Some 20 died in the effort, mostly when explosives detonated prematurely.
The tunnels average about six feet high and four feet wide. Water about 18 inches deep flows along the floor. It gets there from hundreds of holes drilled in the top of the tunnels to allow the saturated soil above to drain.
One can only imagine how many teenagers and older people who should have known better explored those tunnels over the years. Today, most of the entrances are sealed. Those that remain are kept a secret and for good reason. As one gets further back into the system, breathable air decreases, creating a hazard to anyone who enters.