Cave diving to -312m

Yes, that's it! The question mark was supposed to indicate the need for imagination. Over to you . . .

OK, time for a more substantive hint. The clue is perhaps a bit indirect - in fact if there were such a place as "Fringe Pot" the phrase...

It's not easy to come up with a hint without giving this away. For now, I'll just suggest that the letter count should narrow down the...

I think the OMG was a bit of a red herring. Then Spock is an original Star Trek reference (although strictly he was only half Vulcan)...

I suppose the obvious answer would be Vulcan Pot.

That's it - over to you . . .

Market cavities in fish parts? (4, 4, 5)

I'll go with a bit of a guess, maybe Valentine Hole?