There has been a lot of theories concerning what's in the crypts of Winterfell, but I have not found any threads about obsidian (please link below if you have).  - So, in the books we are told that there are hot springs beneath Winterfell, and that this is what keeps the castle warm, even in winter. And what are hot springs? Well, they are heated by the Earth's mantle. This can happen in volcanic areas, where the water is heated by magma. - Obsidian is an ancient form of volcanic glass. In t