That's the concrete and steel tube to the left that the bomb was lowered down. The red container is where all the bomb controls were.
Explosion creates a huge underground cavity that the earth fills in.
There is only one of these on public land. Project Faultless in the Nevada desert. I visited there and found a huge crater with the concrete tube extending about 15 fest in the air in the middle.
If you want to see the lunar landscape these tests create then google Nellis Bomb Range
