You don't built it underground, you build it floating in the water (anchored to the sea floor). Build stretch into the tunnel, and then expand it every $foo years as needed.
ice would be a huge issue in this. thats the big impediment to bridge building in the arctic north.
ambiguity above. underwater but not underground. So beneath the ice but above the mud, silt, and rock so no tunneling problems.
I'd imagine California and Japan have given a decent amount of experience with that.
Earthquake resistance, yes. But building a tunnel right across the middle of a spreading mid-oceanic ridge? Nobody has ever done that before and it seems problematic.