Phnom Sampeau empties more than a million bats into the sky every evening of the year, in a black ribbon that keeps unspooling for half an hour or more. The hill is a lone limestone tower about 130 metres high, rising from a dead-flat paddy plain twelve kilometres southwest of Battambang. Its Asian wrinkle-lipped bats weigh some fifteen grams each and fan out as far as fifty kilometres to hunt planthoppers, the insect that can wreck a paddy field. Researchers reckon the colony spares enough rice a year to feed around twenty thousand people. That makes this cave mouth a piece of Battambang's farm machinery as much as a spectacle. The same rock holds the province's darkest memory: from 1975 to 1979 the Khmer Rouge used a cave near the summit as an execution site.
The summit is where most people spend the last hour of daylight. Up there stand a golden stupa from 1964, a Buddha cut into the cliff and abandoned at the head, two old artillery pieces, and macaques working the offering plates. Steps drop from there into the killing cave, where daylight still falls through a hole in the ceiling onto the memorial below. Then you wait in the field at the foot of the hill, over paddies that run green in the monsoon and gold when the rice is cut from November. Around half past five the first bats trickle out, and within minutes the trickle is a rope you can follow for kilometres. The colony is a fraction of what it was. The community that guards the caves counts roughly a million bats, against five to six million across six district caves in 1999. The river still pours out, every night.