Just off the coast of Massachusetts is the tiny island of Nantucket — where some of the nation's wealthiest individuals own summer vacation homes, from Google's Eric Schmidt (net worth: $13.8 billion) to former GE CEO Jack Welch.

Sotheby's International Realty

There's also "Harbor Hill," a waterfront vacation compound on the island, recently listed for $42.5 million.

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From a bird's eye view, the 18,000-square-foot residence looks like a cluster of 12 separate cottages. "Each structure is very reminiscent of classic New England architecture," says Sotheby's International Realty broker Donna Barnett.

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But one step inside, you'll observe that each "cottage" is actually a different room within the mega-home, connected by enclosed breezeways. In other words, there's a structure just for the great room, dining room, kitchen, master bedroom and so on.

Source: Sotheby’s International Realty

It's also a smart home: "Each room has an iPad that controls everything: the TVs, the heating, the cooling, the music, the lighting," says Barnett. "The technology alone is worth is $2.5 million."

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The owner, a telecom entrepreneur based in Palm Beach, Florida, has his high-tech touches everywhere — in the master suite a covert that TV rises up from the floorboards. When the owner wanted to watch an even bigger screen, he would head down to the mega-home's state-of-the-art subterranean movie theater.

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"It's unlike any other movie theater on the entire island," Barnett says. While most homeowners would probably conceal the hardware and wiring in their homes, this tech-loving owner was so proud of the "brains" of this house, he had it all enclosed in an air-conditioned glass room and lit it up like a giant work of art.

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The 23-room summer compound also has two separate cottages out back — one for the pool house and the other for the guest house — with a private gym, steam room, and sauna.

Source: Sotheby’s International Realty