One of the most unique destinations in the entire National Parks system, Dry Tortugas National Park (www.nps.gov/drto, sunrise–sunset daily, Bush Key closed Feb.–Sept., seven-day pass $5 adults, under 17 admitted free) is a group of seven tiny isolated islands located about 70 miles west of Key West.

Named the “Dry” Tortugas due to the absence of fresh water on the scrub-lined islands, these islands were used for most of the late 19th and early 20th centuries as the southern edge of the United States’ naval defense strategy; accordingly, the heart of the park—and the sole indicator of human habitation—is Fort Jefferson, one of the largest and most remote coastal forts in the country.

Taking over the entirety of Garden Key, Fort Jefferson was constructed in the mid-1800s as an adjunct to a lighthouse built two decades earlier as a means to ward off pirates. The addition of the fort was intended to bolster the Navy’s presence on the island, but eventually the difficulties with getting supplies (and fresh water) to the island proved too much for an ongoing mission here.

By the 1930s, the fort was decommissioned and turned into a national monument. The fort is the largest masonry structure in the western hemisphere, with over 16 million bricks being used in its construction. Ironically, despite all that, the fort remains technically unfinished (several ancillary areas of the fort were never completed).

Although the fort is the only man-made sight in the Tortugas, if you’ve undertaken the boat voyage here, you’ll probably want more than an afternoon sweatily traipsing around the old fort and lighthouse, regardless of how impressive they are. The majority of visitors to the Tortugas use their time here to explore the abundant marine life; expansive coral reefs are home to blindingly colorful tropical fish and the predators who feed on them, as well as lobsters, anemones, sea turtles, and more.

Dry Tortugas National Park contains a recently established 46-square-mile Research Natural Area—basically a well-protected “no-anchor zone”—that provides endless opportunities for exploration in the clear blue waters and helps maintain this fragile ecosystem. There are also ample fishing and boating opportunities in and around the Tortugas, and camping is available at an eight-site campground located near Fort Jefferson. The National Park Service limits the number of outfitters who can provide services—dive charters, fishing trips, eco-tours, etc.—within the park; check the park’s website for an up-to-date list.

Getting to Dry Tortugas National Park

The only way to get to Dry Tortugas National Park is by boat. The two authorized ferry operators are Sunny Days (305/292-6100, www.drytortugas.com, $135 adults, $90 children) and Yankee Freedom (305/294-7009, www.yankeefreedom.com, $159 adults, $149 students, military, and seniors, $119 children), both of which operate out of Key West.