plunge pool

plural plunge pools

1. A pool that is small in diameter, but deep, usually resulting from erosion by a natural force such as a waterfall or rapids.

   By about 450,000 years ago melting glaciers had created a gigantic lake to the north of the chalk ridge, which filled until water began to pour over in a series of cascades so immense that they created ʻplunge poolsʼ up to 140 metres deep at their bases.

   2018, Tim Flannery, Europe: A Natural History, page 205

2. A type of very deep swimming pool.