It's not unusual for luxury yachts to have a submarine in their tender garage. It is unusual for the yacht itself to be a submarine. Purely a concept, for now, the Carapace is the creation of Italian naval architect Elena Nappi. The 256-foot vessel would be powered by diesel-electric and fuel cell systems and would be capable of diving up to 985 feet under the surface for up to 10 days at a time. Spread across the three deck levels are VIP cabins, a lounge, a spa, a bar, and a fitness area, all enclosed so as to ensure access when the open air is hundreds of feet above.