Syri i Kaltër is rain coming back out of a mountain. What falls on Mali i Gjerë, the 440-square-kilometre limestone massif behind the spring, sinks through its fissures and is shoved into daylight here, the strongest of some 18 springs along its foot. It surfaces at about 10°C in every month of the year and boils the pool's middle into a churn that never stops. Divers have gone some 50 metres down the shaft at the centre of that churn and found only more water; the bottom has never been reached. The colour is simply depth: near-black indigo over the shaft, clear turquoise across the pebbled rim, the dark pupil and bright iris that named the place. The water leaves as the Bistricë and runs 25 kilometres to the Ionian Sea. You come the last stretch on foot, under oaks and old planes, and the air turns cold and damp before you see any water.
This is karst, and nothing holds still. The spring gives about six cubic metres a second in an average year, climbs towards eighteen after the winter rains, and in the drought summer of 2004 it stopped altogether. Heavy rain flushes sediment out of the cave system and the pool runs brown for days. On a clear summer noon, though, the sun drops straight into the shaft, the colour comes up almost too saturated to trust, and the coach park is full by mid-morning. Theth, up in the northern Alps, has a small spring of the same name; this is the one people mean. The one thing you cannot do here is swim. When the site was raised from natural monument to natural park in January 2022, swimming in the spring was banned to protect it, and wardens turn back anyone who tries. People cool off in the river downstream instead.