The bomb dropped was a 100-kiloton small-yield tactical nuclear bomb, which was detonated through a missile drilled into the tunnels beneath the Honghu Region's lakebed, hence no mushroom cloud rose on site.
However, this does not imply that the power of this nuclear explosion was small; quite the opposite, as all the explosive energy was contained underground, and the harm caused by the nuclear explosion was entirely absorbed by the subterranean alien creatures.
The high temperatures from the nuclear explosion directly melted the already frozen Honghu, with large amounts of ice immediately vaporized into steam. The flames and shockwave from the explosion blasted a large crater at the original lake bed of Honghu, and the surrounding soil was scorched, turned into a glass-like material.
