Random earthquakes. Same effects as drums of earthquake, except not triggered by anything and not necessarily centered on the hero or anything special. Earthquakes (from any source) now can collapse diggable walls (turning them into floor spaces without pits) and drop rocks and boulders out of the ceiling. No effect in any Plane except Earth. Possibly, to limit this, a finite number of earthquakes can happen on any given level (after which it either collapses entirely, or just fails to produce more free rocks and boulders.)