I realize your remark about an underground nuclear test is in jest, but I thought I'd provide some info about a real one.

The largest underground nuclear test by the United States was the Cannikin shot of Operation Grommet, detonated on November 6, 1971 on the Island of Amchitka in the Aleutian Islands.

The device was a "proof test" for the Spartan missile, which was intended to intercept incoming warheads at high altitude by a large flux of X-rays.

The test was 5875 feet underground.
Yield was just under 5 megatons.
Body wave magnitude 6.8
Surface wave magnitude 5.7

The surface at ground zero was thrown 20-25 feet into the air.

The subsidence crater which formed 38 hours later was 1.5 miles across and 55 feet deep. The final collapse of this crater had a body wave magnitude of 4.9.

Parts of the island and surrounding seafloor were permanently uplifted 4-6 feet.

All info from declassified summary film: http://www.youtube.com/watch?v=OMmFy4NdA8s

The embedded video below is apparently taken from the film Trinity and Beyond narrated by William Shatner, which shows the most interesting parts of the above film.

Brian