Was the cavity dug vertically like that for the test, or was it filled in and the bomb mostly blasted upward?
The bomb blasted upwards for that test. There are a lot reasons for that. Maybe soil composition of the blast site and the yield of the blast. There was an underground test in Nevada where the blast caused the entire surface to fault.Literally the entire surface cracked and uprooted paved roads. In the Cannican blast, it caused the entire island to be raised several feet and cased a 5.0 magnitude earthquake. Usually in most blasts, the explosion causes a molten sphere to form.
With nuclear tests all sorts of weird things to happen. When the U.S. first tested the Hydrogen bomb in 1953, the blast had a yield of 10 megatons. The blast completely vaporised the test island and cause a 30 foot deep underwater crated.