Why can solid-looking ground suddenly collapse in a limestone area?

Answer: Groundwater dissolves rock below until the overlying ground loses support

Groundwater dissolves rock below until the overlying ground loses support ✓ — Slightly acidic groundwater can slowly dissolve limestone and enlarge underground openings. Soil or rock above an opening may remain in place until it can no longer support itself, then collapse suddenly. Heavy rain or changes in groundwater level can help trigger failure.

Tree roots push the bedrock upward until the surface splits open — Tree roots can disturb shallow soil, but pushing bedrock upward is not the mechanism of this collapse. The important change is loss of support below as rock dissolves or material moves into an opening. A normal-looking surface need not reveal that hidden loss.

Rain makes solid limestone swell until it bursts apart — Limestone does not form these collapse holes by swelling and bursting when wet. Water gradually dissolves the rock, and the overlying ground can fall into the weakened space. Not all ground depressions have this cause; the rock type and underground structure matter.