A stunning, two-tiered waterfall made famous by poets, artists and the legend of a ghost dog.

A mesmerizing reservoir nestled at the heart of a pre-Islamic Yemeni village.

A seaside town with no sea in sight.

Underwater art project will double as a massive artificial coral reef to help regenerate the damaged ecosystem.

A plaque marks the underground site of one of London's "lost rivers."

The world's brightest bioluminescent bay.

A flooded Mexican cave that hides a river beneath the water.

Natural rock water slides hidden in the remote forests of northern Idaho.

The glowing blue Nymphaeum of Emperor Tiberius.

This section of the Kennet and Avon Canal has been designated the same level of heritage protection as Stonehenge.

This stunning mountain lake was named for a pair of tragic lovers whose statues stand at the center.

This lonely cast-iron structure is the only wave-washed lighthouse of its kind left in all of Britain.

Enormous lake at the bottom of a unique cave system.

The largest salt lake in the Sahara is mysteriously alluring.

This popular cave pool is truly magical, despite the crowds.

Three sinkholes filled with warm, crystal-clear water reached by a horse-driven cart on an old train track through the jungle.

You may get stranded and everyone's peeing in the water, but at least there's cocktails!

A restored 1920s cascade stairway provides one of the best views of Oakland's Lake Merritt.

Forgotten and then destroyed by fire, this was once one of the largest saltwater swimming pools in the world.

This neon blue river gets its color from a unique mix of bacteria and volcano minerals.

A mineral spring that became all the rage in 17th-century England.

The longest and tallest aqueduct in Britain can actually be sailed across.

A tunnel with windows, so you don't miss out on the impressive views.

Colorful sand dunes create a unique attraction in this part of Utah's red-rock country.