Nobody Understood Why These Squares Appeared, Until This Man Notices One Detail

The hatch

John gripped the wheel and heaved. At first it would not budge, stiff with pressure and salt. He braced his flippers against the steel and tried again, throwing all his strength into it, and slowly, grudgingly, the wheel began to turn.

With a dull metallic groan, the heavy hatch swung inward into darkness beyond.