A mysterious shape catches the eye in this 16th-century painting.

This macabre painting of a Shakespearean scene looks like something out of a nightmare.

The commemorative stone column conceals a secret laboratory.

A tiny replica of the iconic London landmark hides on the side of the steel structure.

A sculpture outside London's oldest cathedral honors the Mohegan chief who died while petitioning the king.

An unusual ode to the late Irish playwright, complete with a squiggly bust and coffin-shaped bench.

This Viking gravestone carved in a rare runic style was dug up from a London churchyard.

The Tudor-style cottage in the middle of the square is not quite what it seems.

Murals of the wild parties once held on the frozen River Thames.

More than a million titles fill the labyrinthine shelves of this prestigious private library.

The last remaining example of the boatmen's perches that once dotted the south bank of the Thames.

Never actually a tree at all, this spot was the site of London's public hangings for nearly 600 years.

A statue of the legendary Celtic warrior queen who fought the Roman invaders stands in one of the cities she once destroyed.

These official units of measurement—including chains and perches—lie hidden beneath tourists' feet in a popular London hub.

This pump on the bad side of Victorian London betrayed a horrifying revelation about the water supply.

On October 17, 1814, a 15-ft. wave of beer ran rampant through the streets of London.

The first tunnel to be dug beneath a river, repurposed as an art and museum space.

A plaque hangs near the execution place of the Scottish Independence leader famously depicted in "Braveheart."

English antique dealers with a taste for the unusual.

Catacombs turned marketplace.

Wedged between two buildings at the Great Ormond Street Hospital is this secret music installation.

A charming 17th-century alley is lined with secondhand bookstores and antiquarian shops.

An unusual round church in London with a Templar past.

An easily overlooked stretch of ornate window fronts and gaslight lamps that could be right from the pages of Dickens.