This was on in the afternoon back in the late ’80s (I’m guessing), and I don’t think it was feature-length, so I assume it was some kind of “Afterschool Special,” although it doesn’t really fit that mold in that there was no real “moral.”
Anyway, it was about a teenager(s) who discovers a really old secret room, maybe in his basement, maybe through a little tunnel. Eventually they discover a skeleton in it (scary!). I think it maybe collapses at some point, or the tunnel in/out collapses, so they’re stuck. But they discover that there’s another entrance at the base of a huge tree on their property.