I was able to stop off at the Hoover Dam while in the Nevada/Arizona area back in May. I was struck by the enormity of these overflow drains. There’s one on each side of the dam and they provide a mechanism for the rapid egress of water which would otherwise lead to dam failure.

Apparently they were necessary once several years ago during an especially rainy time of the year. Otherwise it’s only been used during regular tests.

It’s difficult perhaps to understand the scale of these things (And because the railing there in the background is in a controlled area and the cops, er homeland security officers, were quite interested in why i was standing there so long) it would be possible for a full-sized tour bus to disappear down that hole without coming close to touching the sides or top