Millennium Falcon, or perhaps the relic from “Sphere”? 
I think they messed up something with this story. It says the thing’s at a depth of 300 feet, i.e. quite shallow. You don’t need much, relatively speaking, in the way of fancy equipment to investigate something at that depth.
For what it’s worth, my money’s on a geological formation, probably a mesa-shaped igneous intrusion.
If nothing further is heard about this, you can be sure that it has a perfectly natural explanation. The woo-woos will just move on to the next “mystery” in their unsinkable hope that somewhere among all those misses there’s a hit.
'Luthon64