Agua Azul is a staircase of turquoise pools in the jungle of Chiapas, in southern Mexico, and the river built every step of it. The water comes out of the limestone carrying so much dissolved calcium carbonate that at each drop the mineral hardens out again into a pale dam, which the river then falls over. The same mineral gives the colour: fine particles suspended in the water scatter the light into that blue. It keeps working on whatever falls in, so along the banks you can find logs and roots slowly turning to pale rock inside a growing shell of limestone. A path of steps and walkways carries you about a kilometre up the bank, and each bend opens onto another cascade pouring over a white rim.
You can only swim in the calm basins above the stalls, where the river slows behind a dam into cool, milky-blue water under the forest. The main cascades hide strong currents, slick ledges and dead wood under the surface where you cannot see it. The colour is not permanent either. From November to April the river runs low and the turquoise is at its most vivid. The summer rains swell it into a silt-brown flood that nobody swims in, and even in the dry months a hard overnight downpour can cloud it for a day. At first light mist still lies on the water and the low sun lifts the blue out of the pools. Toucans, parrots and motmots cross the canopy over the walkways, with anteaters and boa constrictors in the forest behind them. All of this stands on the land of Ch'ol and Tzeltal Maya communities, who have farmed these valleys for generations and run the path, the gate and the stalls along it. When an earthquake in 2017 shoved the river out of its bed, they cleared the new channel themselves, with picks and shovels, until the turquoise came back.