You can hear the tell-tale sound of moving water below you, and you peer into the stairwell. It seems further up than you anticipated. You go down to check, and you have to stop before you get to the first floor – the water is still welling up from below, and is at least knee-deep on the next landing down. How it could have reached this high up? You shine your torch down and watch the rolling, churning water for a few moments, then retreat.