Korpovo caves (really abandoned mines, where glass sands were extracted) are one of the most out-of-the-way underground locations in Leningrad region. Once Mutabor, Mokele and Vasily (with all his big family) had organized an expedition to that place. Finally they had found the cave itself, and also a small cave on the opposite bank of the Obla river. This cave can be a natural one and need to be explored further.