Ik Kil is an almost perfect circle punched into the Yucatán limestone, about sixty metres across, with the water twenty-six metres below the rim. Long roots and vines hang from the edge all the way down to the surface, so the shaft is curtained in green. It formed when the roof of a flooded limestone cavern collapsed and opened the cave below to the sky. There are more than ten thousand of these sinkholes across the peninsula. The Maya held them sacred, and Ik Kil was a place of offerings to Chaac, the rain god. Bones and jewellery have been recovered from its depths. It sits about three kilometres from Chichén Itzá, in the same ritual landscape as the great pyramid.
You reach the water down a stone stairway cut into the rock, and the change is abrupt. Tropical heat gives way to cool, damp, echoing air, and a single shaft of daylight lights the pool from inside. Around noon the sun drops straight through the opening and turns the water a luminous blue-green. Black catfish drift through the clear depths and swimmers' voices bounce off the walls. In the rainy season thin waterfalls spill over the rim and run down the roots. The Red Bull Cliff Diving World Series set its 27-metre platforms at the rim here in 2010, 2011 and 2014, though most visitors float in life vests or jump from the lower ledges. Ik Kil is now a developed park with a restaurant and cabañas, and one of the busiest stops on the whole Chichén Itzá circuit. It is quiet at opening and packed with tour groups by midday, which is the opposite of the light you want.