Sumidero Canyon is a rift thirteen kilometres long in Chiapas, in southern Mexico, where limestone walls stand a kilometre over the Grijalva River and leave you a strip of sky. Shared launches run from the pier at Chiapa de Corzo, an old colonial town, and within minutes the walls have closed over you. After that the engine's echo is the only sound in the cool shade of the rock. Almost everything worth seeing here is fixed to those walls. Partway in, water trickling down the cliff has built a broad moss-green formation shaped like a fir tree, the Árbol de Navidad. It runs fullest in the rainy months and thins to a bare mineral streak by late winter. Further on, the boat noses into grottoes at the foot of the rock, chief among them the Cueva de Colores, where iron and copper have stained the stone red and green around a small altar to the Virgin of Guadalupe.
At the waterline the canyon comes alive. American crocodiles lie hauled out on the mudbanks with their jaws held open to lose heat, herons and cormorants work the shallows a few metres from you. The still green water you are crossing is younger than it looks: a dam finished downstream in 1980 backed the river hundreds of metres up the gorge, and only that makes the full length runnable by boat. The walls carry the region's memory too. Local accounts hold that rather than surrender to Spanish forces in the sixteenth century the Chiapanec threw themselves from these cliffs, and the boatmen still tell it as they pass the place. Chiapas has since put the canyon on its coat of arms. The other half of it is seen from above, where a rim road links lookouts that stand straight over the drop. From up there the Grijalva is a green thread a kilometre below, vultures turn beneath the railing, and the boats on the water are too small to pick out.