GIANT cracks have opened in one of the world's tallest dams, just months after completion. The cracks appeared after a tunnel collapsed on 20 June beneath the 200-metre-high Campos Novos dam in southern Brazil, and the reservoir rapidly emptied. At one point, 4000 cubic metres of water (more than enough to fill an Olympic-size swimming pool) were rushing downstream every second towards a second dam on the river Canoas. "If this had happened during the rainy season, and the two reservoirs had been full, water would likely have poured over the lower dam and it might have been destroyed. That would have been a major disaster, with perhaps hundreds killed," says Patrick McCully of the International Rivers Network, a California-based group that campaigns against large dams. Between them, the two dams can hold more than 2 cubic kilometres of water.The dam's owner, Enercan, a consortium of Brazilian power companies, has revealed little about the accident. It is reported to have been trying to patch holes in a leaking tunnel since October, before the second tunnel failed catastrophically last month.