Abismo Anhumas, Natural cave with underground lake in Bonito, Brazil.

Abismo Anhumas is a vertical cave whose entrance opens onto a shaft about 80 metres deep, at the bottom of which lies a lake. The chamber holds stalagmites that grew from the floor when it was dry, including cones up to about 20 metres tall, and a large cone near the water surface. Divers and visitors descend by rope to reach the lake.

The entrance was known to local guides before 1984, when systematic exploration began; the shaft drops about 72 meters to the lake floor. In 1999 the site opened to visitors under a quota system, with descents by rope and a guided route across the lake.

The name Abismo Anhumas comes from the Guaraní language and refers to the depth and water of the system. Visitors experience this linguistic connection to the landscape as they descend into the cave.

Access is by guided tour only, with a fixed daily cap of 18 visitors, so book well ahead. The descent uses a rope rappel of about 72 metres into the chamber; the climb back out is done by a metal staircase and a short fixed rope section. Bring swimwear and a change of clothes, since the lake is entered by snorkelling or diving.

The chamber's submerged stalagmites grew when the cave was dry and were later flooded by the lake. Divers descend about 72 meters by rope to reach the water, and the lake's surface lies roughly 60 meters below the entrance.