With land getting so crowded, the age-old fantasy of sea-based living is becoming reality.

The idea isn’t so far-fetched. As Earth gets increasingly crowded and polluted, some 225 million square miles of prime real estate representing 71 percent of the planet’s surface is largely unused. It’s remarkable considering the oceans promise plenty of living space, fresh seafood, entertainment, and desalinized water. Surely, technology can make this happen.

The most tangible signs are two altered versions of GM’s Hotel Atlantis, at least one of which could be open for business next year.

The first is Hydropolis, a $500 million-plus, 220-room hotel under development near Dubai in the Persian Gulf. Billed as the world’s first underwater hotel, the Hydropolis will be located, if all goes according to plan, 60 feet below sea level and cost $1,500 a night. Among other amenities, the Hydropolis will also feature a missile defense system to guard against terrorists, a shopping mall, and three bars.

Then there’s Poseidon Mystery Island, a $200 million development off the coast of Fiji. When it opens in mid-2008, the hotel will be much smaller than Hydropolis and almost twice as expensive to visit. But it does boast something you don’t get in Dubai: 24-hour views of one of the world’s liveliest coral reefs.

Whether these two projects ultimately swim remains to be seen. The engineering challenges aside, there are plenty of unanswered questions about the environmental impact of these underwater havens. And while Poseidon is taking reservations, construction delays at the Hydropolis will likely extend its scheduled 2008 opening.

CNN Money