Dry Tortugas National Park

Seventy miles west of Key West, at the far end of the Florida reef, sits a cluster of small sand islands, and on one of them stands Fort Jefferson, an enormous hexagonal fortress of some sixteen million bricks begun in the nineteenth century and never finished, because rifled artillery made masonry forts obsolete before it was done. It is the largest brick structure in the Americas, and it sits improbably on an island you can walk around in twenty minutes, moat and all. The water around it is clear and shallow with coral heads and a wreck within snorkelling distance of the beach, and the islands are a critical nesting site for sooty terns and a landfall for migrating birds. There is no fresh water, no shop and no phone signal.

What you actually see

• Fort Jefferson - the vast unfinished hexagonal brick fort and its moat wall.
• Snorkelling off the beach - coral heads and a shallow wreck a short swim out.
• The bird colonies - sooty terns nesting and migrants passing through.
• The night sky - camping out with no light pollution for seventy miles.

When to go

December to April has the calmest crossings and best visibility; summer is hot with a hurricane risk and rougher seas. The ferry sells out weeks ahead in high season and is cancelled outright in bad weather. Camping permits are very limited and must be arranged in advance.

Honest expectations

Getting there is the whole difficulty and expense: a two-and-a-quarter-hour ferry each way or a costly seaplane, and both are weather-dependent, so build a spare day into a Keys trip. There is nothing on the island, so everything including water comes with you if you camp. The day-trip window ashore is only a few hours.

Nearby

Key West is the departure point; the rest of the Florida Keys run back to the mainland.