In the ocean.

The entire disaster was most of the rock the city was built on falling into the ocean, so the ruins are in the ocean.

The disaster occurred around the time of Red Mountain's eruption, which was at least a century ago. Anything wooden would be long gone, stonework can be found trapped in some ice nearby, but for the most part I would assume it has been buried.

If you notice, the college has a bridge, that bridge is as old as the rest of the college, suggesting that the city was once built on the same Ice-formed plateau as the college. Therefore it can be assumed that the collapse utterly destroyed the land beneath the city, burying it beneath the sea bed.