Ghost Pirates of the Sea-erra Madre

You eject from the submarine! The water feels cold, even through your tight sea suit. As you watch, the submarine is torn to shreds by many large tentacles. Thank goodness you got out when you did.

You swim away, to avoid being noticed by the terrible Kraken destroying your sub. As you swim, you notice the water is getting warmer, and there seems to be a light coming from somewhere below you. You swim towards the light, until you see you have reached the ocean floor. Sitting on the ocean floor, surrounded by the faded bones of countless dead sea creatures, is a large glowing cube. It's roughly three feet long on each side, with no markings of any kind on its opaque walls, and it is emanating a warm yellow light from within.