There was a rumble of thunder that rolled around the plateau, silencing the birds and causing the sloths to stir in their perches.

"Rain coming," our guide said. "We must seek shelter."

I remembered what the last downpour that had caught us had been like. It had come on suddenly, the rain falling in great sheets. The rainforest canopy had held the mass of water back for maybe a minute before giving way. Torrents of water had poured from the leaves and off the branches; the ground had become waterlogged, and then fast-flowing streams had almost overwhelmed us. Our clothing had taken days to dry completely, and we had suffered from rashes and chafing of the skin. Fortunately we were in the precincts of some long-abandoned city. Walls of hewn stone, covered in vines and moss and mould, rose up around us. Overgrown shells of ancient buildings were everywhere, offering the possibility of some relief from the weather.

"Here!" McCausland called out and pointed to an opening that was framed by broken stone lintels and thick roots. The three of us hurried towards the opening and reached it just as the first, fat raindrops arrived.

Once through the opening, we found ourselves in a dark chamber that smelled of rot and damp. We didn't care though. We were dry - for now. Water trickled down through gaps in the stone flags above us, but it was nothing compared to what was outside. As our eyes grew accustomed to the gloom, we began to perceive the details of what was around us. Our shelter had obviously once been a cell of some kind. Its last occupant - now a desiccated corpse - was slumped against one well. Only the corroded chains that had been fastened to the wall kept it from falling into a heap. Beside the corpse was a cauldron of some description, its metal rusted and riddled with holes. We had no idea what its purpose was, but there was something unsettling about the way it had been placed in front of the enchained corpse.

A flash of light came from outside, blinding us momentarily, followed by another thunderclap. It would be a long time before the rain stopped.