The eclectic seaside sanctuary caters to a rather fowl crowd.

Two geological phenomena in one precarious locale: a towering stone window, and a deep blue void.

After 400 years of being closed to the public, this monastery's underground tunnels and secret garden are now a museum.

Abandoned Roman Catholic chapel abused by Satanists before being restored.

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

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

Hidden beneath a restaurant, underground tunnels tell the story of Malta’s tragic role in World War II.

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

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

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

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

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

A centuries-old forgotten underworld lies beneath Malta.

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

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

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

This Stone Age solar temple aligns with the sun on each equinox and solstice.

An abandoned set from the 1980 film "Popeye," claimed and repurposed as a theme park by creative locals.

This historic building displays the remains of a victim of the devastating 14th-century plague.

Tombstones and enigmatic inscriptions hide within a vault beneath a popular London arts venue.

This coffee shop lies hidden beneath a historic church.

The signs of a long buried road can still be found hidden beneath a London sewer grate.

This hidden green space is the city's second largest conservatory.

This wildly-tiled artist's palace is a living and growing account of the creator's life, loves, and labors.