What is the largest non-subglacial underground lake in the world?

The largest non-subglacial underground lake in the world is in Dragon’s Breath Cave in Namibia.

Dragon’s Breath Cave is a flooded karst cave in the Otjozondjupa Region of northern Namibia. Its lake covers almost 2 hectares, making it the largest known underground lake that is not beneath an ice sheet. The water surface lies roughly 60 metres below the cave entrance, and the flooded cave reaches considerable depth beneath the surface.

The cave was identified during exploration in 1986 and was later surveyed by teams of cavers and divers. Its name comes from the warm, moist air that can rise from the entrance when atmospheric pressure falls, creating mist that resembles a dragon’s breath. Access is difficult and the cave is not an ordinary tourist attraction.

The qualifier “non-subglacial” matters. Antarctica’s Lake Vostok is vastly larger, but it lies beneath kilometres of ice and is therefore subglacial. The Lost Sea in Tennessee was once widely described as the world’s largest underground lake; it is now generally ranked second among known non-subglacial examples, despite its larger visible surface area measurements in some descriptions.