Doris Duke's lasting collection of Islamic Art, hidden in a Hawaiian paradise.

A tunnel of hardened lava along Maui’s famous Road to Hana.

When Croatia became independent of Yugoslavia, the splintering governments left behind these concrete tunnels.

Formerly Yugoslavia's "forbidden island," this sleepy little city still holds the abandoned remains of military strongholds.

The now-overgrown beach resort was once frequented by Yugoslav military elite.

This scale model of our solar system hid in plain sight for years before a physics team sought out all the planets.

This former estate of a fascist leader is now a burnt out ruin leading to a system of secret tunnels.

A unique two-level bridge designed to honor a king.

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

Just a few miles from Novalja, a strange geological oddity has left scientists baffled.

This military tunnel dates back to World War II and provides visitors with a little wartime adrenaline.

One of the best-preserved Roman amphitheaters is still used today, but for less brutal events.

Now abandoned, the world's first torpedo factory is crumbling into the sea.

A tower marking the spot where Yugoslavia began its resistance against the invading Nazis stands neglected, but not quite forgotten.

Croatia's towering concrete blossom grows from the site of a World War II concentration camp.

Longest fortress system anywhere in Europe.

Gargoyle head that brings good luck.

The complex has been abandoned since the Croatian War of Independence, barring a stint as a set for 'Game of Thrones.'

This truly palatial work of Communist-era architecture now sits crumbling on a Croatian beach.

Once a state-of-the-art tuberculosis treatment spa, these ruins are now ruled by paintball players.

This huge metal monument is slowly being stripped to its concrete skeleton.

Yugoslavia's biggest underground airport lies in ruins.

A wonderfully strange collection of 500+ stuffed frogs doing people things.

This stunning, out-of-the-way Croatian monument commemorates a community's uprising against occupying fascist forces.