World's largest cave was only discovered in 1991, and now offers tours to the public.

A gruesome memorial for those lost in this area during the Khmer Rouge genocide.

5,000 skulls in memorial to those killed by the Khmer Rouge.

The walkable ruins of an 19th century hotel, destroyed in a horse cremation gone wrong.

A careful eye can find hundreds of dinosaur tracks running along the Connecticut River.

Dyed by iron oxide over centuries, these sandstone rocks rise thousands of feet into the sky.

The largest hot spring in the United States is, as the name suggests, a stunning show of natural color.

A beautiful geyser under environmental threat.

One of nature's most well-scheduled phenomenons resides in Wyoming's Yellowstone National Park.

Curious cave that stays a cool 56 degrees all year long.

Thousands of artifacts relating to human sexuality, including the world's largest sex bike.

Rushing water plunges over a 100-foot drop created by an ancient lava flow at this majestic waterfall.

The only structure left with direct ties to the Salem witchcraft trials of 1692.

An absolutely beautiful 200-foot stained glass installation traces the evolution of knowledge.

A sharp bend in a dramatic river canyon cut through red rocks.

Most-visited and most-photographed slot canyon in the American Southwest.

This lake in the Cascades lives up to its name, disappearing down a hole every spring.

While it is not equipped to peer into outer space this observatory looks like it was built on the moon.

This frequently-photographed waterfall takes only a short hike to visit.

Site of the greatest known waterfall that ever existed.

100-year-old shipwreck you can walk up to at low tide.

Smooth basaltic rocks tell a tale of ancient glacial waterfalls.

Thousands of turbines whir at the origin of North American wind power.

A volcanic tunnel illuminated by natural skylights offers a shaded respite from the desert sun.