These remote Florida islands have a history of sea turtles and sunken treasures, and one of the world's largest coastal brick fortresses.

The floor of this spectacular Baroque sanctuary is covered in memento mori.

This almost-forgotten underground dungeon housed several convicts over the years, including Caravaggio.

This subterranean necropolis holds three floors of antler-dug grave caves.

A hidden tunnel of bones in one of Malta's oldest churches.

Stone Age solar temple is aligned with the sun on each equinox and solstice.

Burial grounds for more than 1,000 bodies deep under the modern town of Rabat.

Overshadowed by other, more famous Maltese catacombs, this ancient burial chamber has one of the best examples of a ritual agape table.

Ancient mysterious ruts carved in the bedrock.

These lesser-known Maltese catacombs date back over 1,700 years.

Enigmatic tombs and cave dwellings on the outskirts of a rural village.

At more than 5,000 years old, these structures are among the oldest free-standing stone buildings in the world.

These temples are among the oldest free-standing stone buildings in the world.

This prison on the Maltese island of Gozo is almost 500 years old, and they have the graffiti to prove it.

The fishing boats that moor in this small lagoon enter and leave through a tunnel to the sea.

Dubbed "Wistom" by locals, this abandoned factory is an urban explorer's dream.

This imposing 20th-century castle was abandoned before it was ever completed.

Deep in a Polish forest, this collection of Iron Age megaliths still holds onto many of its ancient secrets.

Countless stories attempt to explain Poland's mysterious J-shaped trees.

One of the largest amphitheaters in Europe is a former Nazi gathering place that still stands today.

This uber-quaint Polish village is known for its habit of painting elaborate floral motifs on just about everything.

Crosses of all shapes, sizes and colors litter the mountain, the site of a huge yearly Orthodox pilgrimage.

A somber memorial honors the hundreds of thousands of lives lost at this labor and extermination camp.

This Nazi ruin was the site where Hitler was almost blown up by a suitcase bomb.