hiding a spaceship

I've been thinking on yet another variation on the "people stumble across a spaceship and get it working" trope.

I'm trying to figure ways this (not alien, but from the future) spacecraft could be hidden yet findable. I figure it should have been hidden several hundred years ago, after getting sent back to the past via [much handwaving]

I figure that about the only practical way is for it to be in an underground location. Probably one that was excavated, not natural. I'm sorta stuck on how to hide the entrance since most of the ways that come to mind would either require gear they wouldn't have or look suspicious to surveyors ("Looks like somebody blasted down that cliff face, Charlie.")

So I'm asking for ideas.

Technology is pretty much that of the Treveller RPG, but with somewhat better computers.