We were letting our imaginations run last night talking with my kids about this. The thought came up that what if a Time Machine located in Seattle in the year 3000 went back 20,000 years? Say around the time between glacial periods and Seattle was just a large river or delta of sediments being laid down. I would imagine that the time machine would keeps it’s lat/long coordinates, but - being subject to the laws of gravity, would end up on the ground surface of 20,000 years ago (say about 80 feet deep).
The poor guy can’t get his time machine back to the future, and is stuck. Only to be uncovered now. I’m guessing that is probably it! (Where is Art Bell when you need him the most!?)
I think the wildest hypothesis is that it's a long buried ancient spacecraft that's made of something too hard for the cutter heads to bite through.
If such a thing were ever to be discovered, wouldn't you know it would be in Seattle? Heh...