Agua Azul is a river that builds its own staircase. The water comes out of the limestone loaded with dissolved calcium carbonate, and at every drop the mineral hardens out again, raising the pale dams the river then falls over. The result is several kilometres of cascade through the jungle of northern Chiapas — pool after pool, each held back by a rim of new stone. Suspended calcite scatters sunlight and gives the pools their name, Blue Water: a milky turquoise that glows brightest in the shallows. The same mineral coats fallen wood, so branches lying in the current slowly turn to pale rock. The falls sit in the municipality of Tumbalá, about 69 kilometres from the Maya ruins of Palenque, which puts the two on almost every regional itinerary. Since 2000 they have been a federal Flora and Fauna Protection Area.
Standing among the terraces, the jungle's wet heat gives way to cool spray and a steady roar as pool spills into pool. A path of steps and walkways climbs the bank, and each bend opens onto another cascade. In the calm upper basins you can wade into water that stays cool and clear. Colour and safety both follow the season. From November to April the river runs low and the turquoise is at its most vivid; summer rains swell it into a fast, silt-brown flood with dangerous currents and submerged dead wood. The falls lie on Ch'ol and Tzeltal Maya land, and you pay both a park fee and a community charge at the gate. They sit off Highway 199, where roadblocks and periodic unrest can disrupt travel, so most people arrive by organised shuttle or tour. Come at first light, before the vans, and the pools are nearly empty in the forest mist.