UST = Underground Storage Tank.
Color me surprised to not have come across one yet in 30 some odd years. Maybe it’s an east or midwest thing.
We briefly considered getting propane, and got estimates - it was assumed the tank would be buried (we’re in S.E. Florida). We opted to go solar instead.
There must be a state someplace that allows burying propane tanks. National code doesn’t allow it. Pressurized tanks have to be periodically pressure tested and re-certified. Even installing a propane tank below grade is normally prohibited because propane is heavier than air and a leak would make an explosive bubble. It’s not even legal to install one indoors. I’ve never ran across a company who would sell a large, installed, tank-—they are leased because of the periodic testing requirement.