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Could be a lava tube. Or a cave structure changed by glaciation. I don't buy into the german UFO base stuff.

My two cents: seismic/ plate harmonic in nature. That said, NOAA does, indeed hear stuff from time to time that they can't nail down. It's a very interesting subject for thought and discussion. I think that saying the Bloop was non-crypto in origin doesn't lessen it's interest.