A hidden alley in urban Baltimore is a haven for graffiti artists.

Quirky castle designed by the American actor best known for portraying Sherlock Holmes on stage.

Elevated freight railway turned wildly successful urban park.

Whisper secrets in a busy station only to be heard on the other side of this acoustic arch.

The site of Paul Revere's historic two lantern warning.

Self-sufficient greenhouse floating on the Hudson River.

This forlorn park is the result of a failed investment turned tax dodge.

The abandoned grounds and derelict greenhouses of this once-magnificent Edwardian estate are now a nature preserve open for exploration.

This abandoned seaside fortress is now simply an imposing ruin being taken back by nature.

The 340-year-old house that inspired the classic Nathaniel Hawthorne novel.

The oldest place in Hartford is a cemetery with thousands of bodies stacked beneath the grass.

Holding multiple historical boats and an entire 19th century village, this New England attraction is the largest maritime museum in the world.

This small-town pizzeria won the Hollywood lottery.

The former home of Samuel Clemens and family remembers the happiest period of the author's life.

This tiny church near Sleepy Hollow contains Henri Matisse's last work, a rose window, and nine windows by Marc Chagall.

The remnants of two World's Fairs are here, complete with a 12-story globe, a mini-Manhattan, and a UFO-shaped pavilion.

The abandoned husk of a mental institution that failed to save a Jazz Age icon still sits nearly unchanged.

This abandoned cemetery is crumbling into ruin, exposing the artifacts and bodies buried there.

This derelict building used to house infirm Freemasons, their widows, and orphans.

Oldest existing church in New York and the inspiration for Washington Irving's "The Legend of Sleepy Hollow."

Decades after testing the polio vaccine on unwitting patients, this historic mental hospital sits in ruin.

America's greatest forgotten garden and the former stomping grounds of Son of Sam.

The home of one of America's most wealthy industrialists is now a historic site that remembers a more decadent time in the country's history.

This once-grand marvel of engineering is now 41 miles of scenic hiking trail.