It's forbidden to dig within this ancient Roman fort hidden beneath a playground.

A massive explosion that leveled hundreds of buildings in the city of Leiden is remembered by a waterside plaque.

Albert Einstein would often sit in this chair during visits to the observatory to discuss science with Willem de Sitter.

This 11th-century castle sits so low you can't see it behind the surrounding buildings.

Graffiti covers the walls of a room where university students through the centuries have waited nervously to defend their theses.

A trio of red, white, and blue stones embedded in the streets of Leiden are markers of medieval territory and law.

Science meets wall art to highlight the beauty of famous mathematical equations.

A market built atop boats is one of the few floating holiday markets in Europe.

A 'living museum' in Leiden, the oldest of its kind in the Netherlands.

A former plague hospital turned medical museum.

Freddie Mercury's childhood home is now a monument to the iconic Queen rockstar.

The anatomical-pathological collection of a man who changed surgery.

Eighty million natural history specimens call this gargantuan museum home.

Kings Cross Station pays tribute to its role in Britain's best-selling book series.

Its sunny hues are believed to have inspired the town's color palette.

In a busy Oaxaca market, a nearly century-old drinks stand still serves family recipes from giant clay pots.

A park in Mexico offers all the experiences of an illegal border crossing except for one: the border.

Iconic red and white plaques show the creative names invented to navigate the city streets.

Few of the many visitors to this resort town know of its hidden Maya ruins.

If you can look past the nearby sewer pipes, this urban sinkhole grotto is a mini-paradise.

Clap your hands at the base of the pyramid, and the song of a sacred Mayan bird will echo through the air.

A uniquely robust and historic look at Mexico's omnipresent dance with Death.

No telescopes, no power, no gift shop: The Observatory of Chichen Itza, one of the world's oldest observatories, takes astronomy back to basics.

Every equinox this Maya pyramid puts on a spooky ancient light show.