What is That Huge Hole in Hoover Dam?

What is that scary and enormous hole? It's the spillway overflow tunnel at Hoover Dam, designed to manage excess water in the reservoir. When the water level in the Hoover Dam reservoir reaches its maximum allowable height, water flows into two enormous spillways located on either side of the dam. The water is then directed through overflow tunnels, which are approximately 50 feet in diameter and 690 feet long, channeling it back to the Colorado River below the dam.