The Dry Tortugas, a cluster of islands 70 miles from Key West and accessible only by boat or plane, offer a glimpse of pristine tropical wilderness. RELATED LINKS City Guide Key West

A day trip to this remote cluster of seven flat islands 70 miles from Key West will put a fair dent in your vacation budget, but if you can scrape together the cash, you won't be disappointed.

The islands, along with nearby shoals, coral reefs and more than 64,000 acres of surrounding water, comprise one of America's most extraordinary national parks. The site offers a mix of excellent diving and snorkeling, beach walks, bird-watching and a chance to explore Fort Jefferson, an impressive feat of 19^th-century military engineering with massive hexagonal brick walls and a saltwater moat.

Two zippy catamarans, the Yankee Freedom and the Fast Cat II , provide passenger service to Dry Tortugas National Park, departing Key West at 8 a.m. and arriving at around 10 a.m. Day-trippers have about four hours to tour the fort, swim, snorkel and picnic before heading back to Key West in mid-afternoon. Overnight visits are possible as well, with a limited number of primitive campsites available in the park.

Seaplanes of Key West offers a faster and more expensive way to get to the park, with five- and eight-passenger amphibious aircraft making the trip to Dry Tortugas in about half an hour. The flight itself is well worth the price of admission, skimming low over the sea for spectacular aerial views of uninhabited islets, shipwrecks and large marine animals including sharks, stingrays and sea turtles. Upon arrival, pilots make a pass over Fort Jefferson before landing. Half-day, full-day and overnight excursions are possible.

For more information, visit the National Park Service Web site.