Originally a huge reservoir carved out in the 5th century BC to supply the ancient city of Akragas with water, fed by a network of underground channels (hypogea) cut directly into the soft local rock, an extraordinary feat of hydraulic engineering for its time.
Once the reservoir silted up and lost its original purpose, later generations planted the sunken space with citrus, olive, and almond trees, and it's remained a cultivated garden ever since, now cared for by the Italian environmental trust FAI (Fondo Ambiente Italiano), which restored it after decades of neglect.
Several of the underground galleries are open to explore on foot, cool, narrow tunnels that once carried water through the hillside. The whole garden stays noticeably cooler than the open temple paths above it, a genuine, shaded break from the sun if you're visiting in summer.