This strange seaside tower got its name from a wealthy eccentric who would hide coins among its stones.

A hidden network of subterranean passageways lies tangled beneath Indianapolis' bustling City Market.

The largest self-built castle in the U.S., six decades in the making.

The longest continuous lava tube in the continental U.S. offers 2.5 miles of dark explorations for the adventurous.

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

This geothermal hot spring hides beneath a slowly growing mineral dome.

This natural wonder drops 16 stories straight down through rare flora and streaming waterfalls.

A self-built castle made by a mysterious man.

Hidden in Central Park's Ramble is a troublesome cave buried just beneath the surface.

Enormous lake at the bottom of a unique cave system.

One man has carved a number of natural New Mexico caves into psychedelic sandstone temples.

A giant Troll lurks under one of Seattle's oldest bridges.

Crystal clear lake with a shoreline of travertine located in a Colorado canyon.

This abandoned subway station beneath the Waldorf-Astoria hotel is allegedly used to secretly transport presidents.

A 140-year-old castle with underground caves hidden in San Francisco.

These remote Florida islands have a history of sea turtles and sunken treasures, and one of the world's largest coastal brick fortresses.

An emerald-green grotto just a short trip from Austin.

Dramatic river bend surrounds a natural red-rocked pedestal.

A crumbling hospital from the 1850s on Roosevelt Island.

A peculiar mansion built by the troubled heir to the Winchester Rifle Company fortune.

During prohibition, corrupt city officials ran drinking dens under the streets of Downtown Los Angeles.

A huge musical instrument played by the ocean.

A beautiful and abandoned New York subway station from 1904, complete with chandelier.

An abandoned railroad tunnel transformed into a creepy path for hikers and cyclists.