The Great Blue "Hole" is actually an atoll. Atolls are a round set of coral that raises to create land. The Bikini Atoll was another famous one (US, 2000 mi SW of Hawaii) that its natives were evacuated to use the site as a nuclear weapon test plant. Way to go tree huggers!!
I bet its an alien space craft. Since its in Florida, the extraterrestrials were obviously getting a nice tan. Who wouldn't? Leaving Mars to come closer to the Sun is more than enough explanation for anyone.
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