Article: Experts map deep magma reservoir below Washington's Mt. Rainier
Mt. Rainier last erupted in the 19th century. It has a large magma chamber under it. It does not appear to be primed to erupt again anytime soon.
Doing some quick math, it appears that while the chamber is 5 miles below the peak, the peak is 2.7 miles above sea level. This puts the magma chamber a little more than 2 miles below sea level.