You cruise silently into the grotto. Its roof seems to slope upward and you follow the slope. The depth finder shows that you are rising quite rapidly. Perhaps you will reach the surface and open air. Then the roof of the grotto stops sloping upward.
Before you is a perfectly round metallic hatch made of a metal that you have never seen before. With the mechanical arm of the Seeker you try to open the hatch. It will not open. You begin to send radio signals at the door hoping to make contact on the other side.
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