The 'basement' you're referring to is the inside surface of a polygon (usually a cube) that surrounds the level. There are usually invisible walls that keep the player camera from falling off the level's surfaces, but sometimes there are cracks or gaps in the walls.I've gotten past the walls in Lindisfarne, Thronheim, and the Afterworld. Others have reported doing so in Thjorgard and Frosgard. It usually only happens if you're exploring in some forgotten area where the designers didn't expect you to go. You usually won't be able to rescue yourself without a transportation spell.
