A World War II evacuation tunnel turned into a history display of Croatian state and people.

A well-worshipped shrine to the Virgin Mary nestled inside the last of Zagreb’s historic gates.

An enormous, inside-out glass globe built in 1935.

One of the now-less-hidden floral gems of the "Emerald Necklace."

One of the last remaining World War II Victory Gardens in the U.S. is quietly growing across from Fenway Park.

Two thousand artifacts from around the world collected by one woman who loved to travel.

This mountaintop lake bubbles and steams in a perpetual boil, thanks to its direct link to the Earth's interior.

Thermal springs underneath the seabed provide the heat and bubbles for these beachside natural hot tubs.

This huge pile of volcanic boulders hides a stunning network of secret grottoes.

A newspaper editor turned this abandoned island into a tortoise nature preserve that's now the world's smallest national park.

This vestige of Seychelles’s colonial past now resides in a busy intersection and didn’t chime for its first 96 years.

This African beach is home to thousands of rare mainland penguins that are known for their annoying voice.

Sweden's first Viking city is also the site of thousands of burial mounds.

This patch of land is home to Sweden's largest burial mound, allegedly belonging to a mythical king.

The largest wine cellar in the world is a 150-mile network of underground caves near the capital of Moldova.

This 13th century Orthodox monastery is built right into the Moldovan cliffside.

Nine centuries-old fortresses guard the walls of this medieval Romanian city.

A fire once nearly devoured this gorgeous 14th-century Gothic cathedral.

The European towns of Brașov and Râșnov each have proud signs reminiscent of the Hollywood sign.

This elaborately decorated, fairytale-like Romanian royal palace is one of the most striking castles in Europe.

This communist behemoth is the heaviest building in the world and a legacy of a brutal regime.

Rebuilt several decades ago, its striking murals prevent it from being reconsecrated as a church.

A striking symbol of the Bulgarian capital.

This fortress dates back to the 6th century and is also the site of a controversial abandoned restoration project.