The construction of Hoover Dam, an impressive concrete wall built across a narrow canyon of volcanic rock, resulted in the creation of Lake Mead which lies in the middle of arid hills and which can be explored via routes laid out in the surrounding desert. Started in 1931, the dam was completed in 1935, when its height of 1 243ft made it the largest dam in the world. In 1999, the dam was recognised as one of the most important construction projects of the 20C.