Where the shinigami recruitsare trained and taught by some of the best sometimes you can expect a captain or two to show up and look in on this and maybe give a lesson.
This chain of underground tunnels that can be entered by many caves all around soul society has been around a VERY long time and its tunnels have surely still not all been explored. The tunnels lead deep within soul society like a maze and if you go too far and get lost, well you can be sure that getting back will be a task of pure luck. The deeper you go the more powerful the creatures and hollows that live in these caves will become. Rumors say that some of the rare "Vasto Lord" class menos live deep within these tunnels.