These are the caverns that have been permanently submerged by the underground river that snakes through Oulus. They are extremely dangerous, and nobody's quite sure just how many caverns there are. Several people have drowned in these already, so they're cordoned off and all the known entrances are kept under some close watch. If a Candidate or weyrbrat is spotted coming close, rest assured, they will be punished. It's a really creepy place to be anyway, and many are superstitious of it. You don't want to go near them, especially not at night.