Lanai, Hawaii

Larry Ellison stood on the white sand of a deserted beach, bordered by palm trees laden with coconuts. It didn't feel real that he could own the island of Lanai, he reflected.

It had been his far-fetched dream since he was in his 20s, when he first flew over one of the smallest of Hawaii's inhabited islands in a Cessna 172 and was captivated by the thousands of acres of pineapple fields.

In June 2012, Mr. Ellison, the co-founder and chief executive of technology giant Oracle , bought Lanai for $300 million from American businessman David Murdock. Now he owns nearly everything on the island, including many of the candy-colored plantation-style homes and apartments, one of the two grocery stores, the two Four Seasons hotels and golf courses, the community center and pool, water company, movie theater, half the roads and some 88,000 acres of land. (2% of the island is owned by the government or by longtime Lanai families.)

For the first time, Mr. Ellison has publicly detailed his ambitious and costly plans for the 141-square-mile island. They include building an ultraluxury hotel on the pristine, white-sand beach facing Molokai and Maui and returning commercial agriculture to the clear-cut acres. He also plans to endow a sustainability laboratory that will help make the island "the first economically viable 100%-green community." And one of his biggest tasks: winning over the island's small, but wary, local population, one whose economic future is heavily dependent on his decisions.