Semuc Champey is a limestone bridge in the jungle of Alta Verapaz, in central Guatemala, and what runs across the top of it is only the part of a river that stayed above ground. The Cahabón drops out of daylight at the upstream end and runs some 300 metres through a tunnel of its own making. Nearly all of it goes under. The small overspill left on the surface is what fills the staircase of pools on the roof of that tunnel. So little water crosses that the basins barely move: glass-clear, warm, stepping down the valley in green and jade behind neat travertine rims. You wade in and swim from one to the next. The limestone is smooth underfoot, small ledges let you drop gently to the level below, and where a pool spills over its lip the falling water makes a curtain you can sit under.
The climb to El Mirador comes first for most people, about 45 minutes up a steep, humid jungle path until the trees open. The whole staircase lies far below from there, stepping down the valley toward the place where the river disappears. Past the end of the bridge the Cahabón bursts back into daylight from under the rock, and inner tubes ride it out through the jungle, faster and wilder than anything above. Just outside the park, guides at the flooded K'anb'a cave hand you a single candle and lead you wading and swimming inside, up rope ladders past small waterfalls in near-total dark. The only light is the flame in your hand and the ones bobbing ahead of it. The last thing to see is the lip where the river goes in. The brown Cahabón thunders into the stone there, and a warden stands at the edge with a whistle, because the current has pulled people in and never given them back. The still water you swam all afternoon is the part of it that missed the hole.