The results of whatever caused the crunching sound are now fairly obvious: there's a large, jagged hole torn through the outer wall, letting light from the setting sun stream inside the library.

What might be a little more interesting, though not necessarily less dangerous, is that the table with the symbol on it, along with the entire floor of the room, has disappeared entirely. In its case, a deep blue staircase leads down into what looks to be an underground room and, beyond that, what looks like a hallway.