Written by someone on a forum:
Back in the day it was of great interest to both the government and citizens as to how effective a ... all » nuclear weapon that were made 15-20 years prior would be. The solution was to simply blow them up and measure the results, but detonating a bomb on the surface was not allowed anymore. So the solution was underground testing.

For these tests to be effective, three things have to happen. Dig a hole, put bomb in hole and blow it up, and then the gather of results.

My grandpa was the second man in charge of step one and three, digging holes. These were not your average holes, they were 24” to 36” in diameter, some up to 48” for a depth up to 2000 feet. As my grandpa was experienced in drilling holes in the oilfield (8-9 inch holes up to 16,000 feet deep) he had pretty applicable knowledge and experience of how one might accomplish this. As they were feeding in pipe (as the hole got deeper), by the time they got 2000” of pipe, the whole string of pipe weighed over one million pounds. By comparison, 13000 feet of 2-7/8” pipe in the oilfield weighs around 100,000 pounds.

So once the hole was dug, my grandpa and his crew had to leave as they placed a building over the hole (for secrecy/privacy) then lowered the nuke in. They then put so much concrete over the top of the device and filled the rest with dirt.