Fort Peck Dam, the largest hydraulically filled dam in the world, was built from a slurry of sand, silt, clay, and gravel dredged from the Missouri River. Nearly four miles long, it required more than 125 million cubic yards of material, enough to fill more than 12 million dump trucks. Huge dredge boats sucked a watery mixture of sediment from the river bottom and pumped it through long pipelines to the dam, where the material settled out of the water and the core solidified. Although it appears to be a mound of dirt, the dam was engineered to withstand the pressure of 130-mile-long Fort Peck Lake. Deep inside, a 35-million-pound steel wall prevents water from seeping underneath the dam; this wall was driven deep into the shale bedrock below and surrounded by an impervious clay core. The force of the water helps stabilize the dam. In August 1935, 200 workers set a world record by driving steel plates more than 160 feet into the bedrock to support the earthen dam, and together these steel walls stretch nearly two and a half miles long.