Lenin is above ground, though his glass case is kept airtight. There are also Catholic saints with intact bodies above ground.
An embalmed body in a vault below ground will retain its appearance for a very long time, though the skin will darken.
Actually, a body in a mausoleum decays much faster, due to the exposure to air.
LOL! Sink, I am just teasing you,
but how do you know so much about
caskets and the like?