Geography rarely got in Henry Flagler’s way. A Gilded Age industrialist who amassed one of America’s great fortunes during the economic boom of the late 19th century, Flagler co-founded Standard Oil before turning his attention and deep pockets to Florida in the 1880s. Over the following decades he pushed his Florida East Coast Railway ever further south, developing grand hotels along the route.

Ultimately, he ran out of mainland. But tantalizingly out at sea lay the Florida Keys, a ribbon of islands that could only be reached by boat. So Flagler did what Flagler did best. He found another way over.

His final great project carried his now long-gone railway through the Keys. More than a century later, it’s easy to see why Flagler was tempted. The islands trail from the southern tip of Florida toward Key West, slivers of tropical green scattered across the brilliant blue of the Atlantic. Today, 42 bridges link them along the Overseas Highway.

At the archipelago’s heart is Marathon, a neighborhood spread across 13 islands. On Fat Deer Key, Coco Plum Drive leads through a peaceful residential enclave to the ocean. At the water’s edge, 2405 Coco Plum Drive occupies 1.7 acres with 200 feet of unobstructed beach. Newly........

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