Possibly a large urban centre.
Up to 6000 years old.
But they’ve found “stone structures” underwater before, that have turned out to be natural formations…
Other links I dug up with reference to this discovery: Underwater City Reported Off Western Cuba (May 18), Update on Mysterious Deep Water Sonar Images Off Western Cuba (June 13), Atlantis: Where is the Lost Continent?, ‘Lost City’ Found off Cuba
(Yahoo link via Invisible Broadcast System)