Nondescript elevator doors open to reveal a stunning, surreal world.

One of the last few places in Manhattan that employed a pneumatic systemic keeps it on display.

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

A Pythagorean funhouse that tries, successfully, to prove that math is the coolest thing ever.

The feral felines of Roosevelt Island have a small army of allies and an abandoned hospital all to themselves.

Elevated freight railway turned wildly successful urban park.

A crumbling hospital from the 1850s on Roosevelt Island.

Beneath Kraków Old Town's main square lies a hologram-filled medieval market vampire graveyard wonderland.

This unattributed sculpture in Amsterdam is hiding in plain sight.

In the days before house numbers, these colorful signs held clues about various buildings' purposes.

One of Amsterdam's narrowest houses is located right across the canal from its widest.

Seven mini rowhouses fill the gap between 54 and 70 Westerstraat.

The world's first zoo for microbes collecting the horrible and wonderful things living on and around us.

A floating cat sanctuary on an Amsterdam canal.

This curious sculpture represents the perseverance needed to take to the skies.

A monument to the man who almost killed Hitler.

Gardens representing parts of Asia, Europe, and the Mid East are tucked away among the Soviet-style buildings in East Berlin.

Art and a pun, all in one.

A museum dedicated to the history of a terrifying secret police is now held in their former headquarters.

A slice of central Asia in the middle of Berlin.

This small museum is basically a time capsule of daily life in the Soviet Union.

A museum dedicated to controversial Russian pianist Alexander Scriabin.

A monument to Malchik, a stray dog who lived in a Moscow metro station until his tragic death.

Some Moscow residents once claimed the bizarre, contorted mass of metal on top of the building was used to manipulate minds.