An architect's childhood fantasy treehouse made real.

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

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.

Amsterdam's former hotspot for public executions and dissections.

Three connected pink triangles memorialize persecuted gays and lesbians throughout history.

This city square is home to Amsterdam's oldest church and a statue honoring Dutch sex workers.

The attic of a four-century-old canal house hides a clandestine church.

Dynastic Home of the De Piro Family.

This subterranean necropolis holds three floors of antler-dug grave caves.

The remains of a 1st-century aristocratic Roman town house featuring beautiful well-preserved mosaics.

Burial grounds for more than 1,000 bodies deep under the modern town of Rabat.

This unique building in the main square stands out among the city's typically Baroque architecture.

This spectacular dome managed to survive after being bombed in World War II, in an event known as the "Miracle of Mosta."

The perfectly preserved underground rooms where Churchill plotted the war against Germany.

A sprawling ancient Jewish cemetery with over 30 rock-hewn burial caves.

An ancient underground cultic temple where sunlight penetrates on the summer solstice.

The bluer than blue waters in these natural Israeli caves are reached via the world's steepest cable car.

Macabre astrological automaton clock dating to the late 1400s.

Synagogue built from the stones of Solomon's temple contains the golem of Prague.

The former Nazi concentration camp stands as a museum to remember this dark chapter in European history.

Biblical beasts and endangered animal preservation come together in this Israeli zoo.

A massive, heart-shaped, wind-carved crater in the middle of the Israeli desert.

Architecture lovers won’t stop touching the National Gallery's 19.5 degree marble prow.