A circular stone pool about four metres deep, fed continuously by warm artesian water bubbling up from below. The Greeks knew the spring as the Spring of the Sun, mentioned by Herodotus; the modern name is touristic licence, with no evidence Cleopatra was ever within a thousand kilometres of Siwa.
It's a relaxed place to swim, surrounded by palms with a small cafe alongside that serves tea and shisha. Mornings are quiet. It can get busy in the afternoon when tour groups arrive on the standard Siwa loop, which also takes in the Temple of the Oracle just up the road.