DRY TORTUGAS NATIONAL PARK, Fla. — Many visitors to the Florida Keys snap a picture of the marker in Key West that designates the southernmost point in the Mainland United States. Or they buy trinkets bearing images of another famous local landmark — the "Mile 0" road sign, marking the end of the Overseas Highway (U.S. 1), which begins more than 100 miles to the north.

But the Keys actually don't end in Key West. Seventy miles farther west lies a national park, the Dry Tortugas, situated on a string of islands in the Gulf of Mexico. The park offers sparkling ocean views, bird-watching and a Civil War prison, Fort Jefferson. And although it's easily reached by plane or ferry for a day trip or overnight camping, it is one of the least-visited of all national parks, with just over 61,000 visitors a year.

"It's not a park you can just get in your car, load up the car and drive to it. It's got 70 miles of ocean between the last point you can drive to and the park," said Bonnie Foist, chief ranger of Everglades and Dry Tortugas National Parks.

But Foist echoed tourists to the park, saying a day trip is worth the effort to see the fort, one of the western hemisphere's largest brick structures.

"It's overwhelming, just as you approach the park and the fort. It's absolutely breathtaking. You're traveling for hours and all you see is ocean and here pops up this beautiful fort," Foist said.

Fort Jefferson is on an island in the Tortugas called Garden Key, which is served daily by two ferries and a seaplane from Key West. Only five parks in Alaska and two others draw fewer tourists, according to an annual national parks survey. By contrast, the 10 busiest parks each draw 2 million to 9 million visitors each year.

"Logistically, it's just a hard place to get to," agreed Wayne Landrum, who was supervisory park ranger at the park for six years and who later wrote a book about it. "Plus, it's expensive to get there."

Ferry tickets cost more than $100 for adults, and another obstacle can be the time it takes. The ferry trip is two hours each way, and day visitors spend approximately 4 1/2 hours on the island. Both ferry services serve breakfast and lunch and hand out free snorkel gear, but other amenities like an on-board bar and showers vary. Traveling to the island by seaplane takes about 40 minutes but costs $189 for a half-day visit.

There is a campground on Garden Key, but campers must bring their own food and fresh water, and remove all their trash.

The islands are renowned for the migrating birds that stop over. In fact, the area's wildlife was one of the first things that struck Spanish explorer Juan Ponce de Leon, who discovered the string of sandy coral islands in 1513. He named the islands Las Islas de Tortugas, or The Islands of Turtles, after the reptiles he found there. The islands later became known as the Dry Tortugas, meaning no fresh water could be found.

After the area became a territory of the United States, fortifications were begun to protect the shipping lanes of the Florida Straits and defend the Gulf Coast. In 1846, the U.S. Army began building the fort.

Construction continued for three decades, but the fort was never finished. The invention of a new type of cannon made the walls penetrable and the fort obsolete.

During the Civil War, the location was used both as a staging area by Union warships and as a military prison. Its most famous prisoners, however, were four civilians, coconspirators in the assassination of President Lincoln. One, Dr. Samuel Mudd, set Lincoln assassin John Wilkes Booth's leg after he fractured it jumping to the stage from the presidential box at Ford's Theater.

Mudd said he didn't know of the assassination plot and was pardoned four years after his conviction. His descendants have struggled unsuccessfully for decades to clear his name by getting the conviction overturned.

The fort was made a national monument in 1935, and the area was designated a wildlife refuge in 1908. In 1992, it was renamed the Dry Tortugas National Park.