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

On a clear day, you can see all the way to the Virgin Islands from the top of this observation tower built in the 1960s.

A magnificent waterfall nestled in the heart of the El Yunque National Forest.

This small primate research island is teeming with playing, fighting wild monkeys.

Rusting, graffiti-covered tanks litter the beach of this island, once used by the U.S. military for target practice.

The world's brightest bioluminescent bay.

On this Puerto Rican island, many abandoned naval ammunition bunkers sit open while a few have been padlocked by locals as personal storage.

The ruins of the first Spanish settlement in Puerto Rico.

Puerto Rico's oldest rum producer started its 140-year-old business with "a little barrel."

A former pyramid-shaped library with colorful murals lies just across the bay from Old San Juan.

Sloth rehabilitation center in the jungle of Costa Rica.

A hyper-remote desert cave dramatized in "The English Patient."

Even King Tut was intrigued by the glass of the Great Sand Sea.

Scattered monuments in the middle of the Sahara dedicated to the late Egyptian actor and desert explorer.

A remote monument to one of the fiercest battles fought by the Victorian British Army.

An epic statue dedicated to the memory of the 9th and 10th Cavalry Regiments of the U.S. Army

An obelisk designed in memory of the soldiers from the city and district who fought in The Great War.

A tribute to the man who helped develop Pune.

An extraordinary "City of Caves" carved from solid stone.

An eclectic mix of influences rot together in harmony in this India necropolis.

"The country that ate itself."

Anyone wanting to know what a dwarven mine sounds like need only visit the tunnels of this New Zealand artillery compound.

Built over an abandoned train yard, this hidden gem is full of winding paths and offers gorgeous views of Sydney Harbour.

People can't get enough of these adorable Japanese photobooths.