Sumidero Canyon is a rift thirteen kilometres long in the state of Chiapas, in southern Mexico, run by boat between limestone walls that climb a kilometre straight out of the Grijalva River, with crocodiles on the mudbanks and waterfalls hanging in moss on the cliffs. The boats leave from the pier at Chiapa de Corzo, an old colonial town a few minutes from the state capital, and the canyon closes in within minutes: the walls rise sheer on either side, so high that the sky is a strip, and the river runs green and still between them. American crocodiles lie hauled out at the waterline with their jaws open to cool in the sun, while herons, cormorants and spider monkeys work the banks a few metres from the boat. The best-known wall is the Christmas Tree, where a waterfall has built a cascade of moss-covered stone terraces down the cliff in the shape of a fir, and the boats nose into the grottoes at the foot of the walls, chief among them the Cave of Colours, where minerals stain the rock pink and green.
The river at the far end is a reservoir, dammed just downstream in 1980, which is why the water in the gorge is deep and still and why the boats can run its whole length. Local memory holds that in the sixteenth century the Chiapanec people threw themselves from the canyon's cliffs rather than surrender to Spanish forces, a story still told on the boats. From the top the canyon is a different thing: a road along the rim reaches a string of lookouts where the river shrinks to a green thread a kilometre below, and the depth of the drop finally registers, with vultures circling beneath your feet. The canyon appears on the state's coat of arms, and the rift that made it opened some 35 million years ago along a fault. Sumidero is a canyon best seen from both ends at once, and the boat is where the walls are tallest.