Mich. Comp. Laws § 125.565: Tanks located underground.

Sec. 15.

Tanks located underground shall have the top of tank at least 3 feet below the surface of the ground, and below the level of the lowest pipe leading into the building to be supplied. Tanks may be permitted underneath a building if buried at least 3 feet below the lowest floor, or they may be placed 24 inches below the lowest floor and covered with 15 inches of earth and 9 inches of brick or concrete.