With considerable trepidation you slide down the tunnel. You mean to turn off at one of the side passageways that might take you back to your own time, but you are too exhausted to search for the entrance. You half slide, half climb down the tunnel-further back into time-then you lose your balance. You fall a great distance, land in deep water, and sink a long ways.

You are barely able to surface and, when you do, you find that the air is filled with dense fog. The water is warm, but the fog is so thick you can hardly breathe. The air smells slightly of sulphur. Gasping for breath, you realize you must be several billion years in the past and that oxygen has not yet been released into the atmosphere.