Here is the bridge over the over flow canal that leads to the over flow pipe.
It should give you a frame of reference for the following shot.
For the length of the bridge (which has a semi on it) is the width of the canal.
This is the over flow pipe for the dam,
now remebering that the bridge going over the canal is longer than a semi
this is a very big hole in the ground.
I hoped you liked my shots of Hoover Dam.