Phnom Sampeau is a limestone tower standing alone on the paddy plain outside Battambang, in western Cambodia, and every evening it pours more than a million bats into the sky. You take one of the plastic chairs in the open ground at the foot of the hill and watch a dark cave mouth. The first bats trickle out just before sunset. Within a few minutes they have knitted into a single unbroken rope that keeps pouring for half an hour, curling out over the paddies and thinning away out of sight. The hill has a legend to match its name: sampeau means ship, and in the Khmer story a royal junk was sunk here by a magic crocodile and hardened into rock. The crocodile is out there too, as another hill on the same plain. The way up threads past gilded gateways and rows of statues built into the karst, with the plain dropping further below you at every bend. Partway along you pass a giant Buddha cut into the cliff and then left: the head finished, the body still raw stone.
The other cave takes you down steps through a gap in the rock. The chamber below is lit from above by a natural shaft in the roof, and that shaft is what the Khmer Rouge used here; when the regime fell in 1979, human remains were found across the floor beneath it. Skulls and bones now lie in a glass memorial under a reclining Buddha. How many died is not known. The figures repeated around the mountain run from a few hundred to ten thousand; the caves were kept as a shrine, tended by monks and survivors, rather than turned into a forensic site. The climb back out takes a minute, and most people make it without talking. By then it is late afternoon, and the field below is filling with chairs. Under the roost, meanwhile, the droppings pile up: villagers collect two or three tonnes a month and sell it as fertiliser for the same rice fields the bats will be working over that night.