Mavin's earthfast

Mavin's Earthfast was a 10th-level Arcanist spell used in the days of Netheril. It was originally created to allow continued mining of areas that would otherwise be too dangerous to mine. Casting Mavin's earthfast made large areas of underground tunnels become impervious to cave-ins and such. Later its use was adapted to preserve important areas of cities.

Spell Components
The components needed for this spell were a stone of controlling earth elemental, six chardalyns (each imbued with mud to rock), and an Earth elemental.

History
This spell can no longer be cast due to the restrictions placed on magic by Mystra after the near cataclysmic destruction of The Weave by the casting of Karsus's avatar in -339 DR.