The level serves as a connection between the four power plants and contains eight turbines underneath the plants themselves, but only four are working. In some sections, water is flowing into the turbines. Touching the water streams will knock the Courier down, but cause no damage. The water is marked as owned, replenishing 2 HP with no rads.
The turbines are marked N1 through N8. The functioning turbines are N1, N3, N5, and N7. Near the convergence of N6 and N7, there is a breakroom of sorts, with tables, chairs, one Nuka-Cola, and two Sunset Sarsaparilla machines. Near the exit from N8, there are two NCR troopers standing guard.
The turbine's numbers correlate to those above in the power plants, with one exception. Although the lower level turbine numbers progress from N1 to N8, the correlating numbers above repeat N3 and N4 in both power plants 2 and 3.
The Hoover Dam lower level appears only in Fallout: New Vegas.
Behind the scenes
The Hoover Dam lower level is based on the lower levels of the real world Hoover Dam, which includes similar walkways, tunnels, stairwells, and a resemblance overall in architecture and function.