In December 1900, the keepers of the Flannan Isles Lighthouse off the northwest coast of Scotland mysteriously disappeared — and to this day, no one knows what happened.

Step inside Marilyn Monroe's house, the Spanish Colonial at 12305 Fifth Helena Drive in Brentwood, California where she spent her last days before dying in 1962.

Starting in the late 1970s, the anonymous Circleville letters were sent to hundreds of residents, local businesses, government offices, and others, threatening to reveal the recipients' darkest secrets — or worse.

At the start of the 1983 Sydney to Melbourne Ultramarathon, Cliff Young quickly fell behind and seemed on track to finish last. But while the others stopped to sleep, Young kept running at his tortoise pace for five days straight — and won.

Between 1948 and 1975, the U.S. Army tested chemical weapons like mustard gas and LSD on American soldiers at Maryland's Edgewood Arsenal facility.

The mistake wasn't uncovered until nearly 50 years later when the now-adult Paul Fronczak decided to take a DNA test and discovered he wasn't related to his mom or dad.

Martha Place brutally murdered her 17-year-old stepdaughter Ida with acid in 1898 and was executed by electric chair the following year.

Between 1979 and 1981, David Carpenter murdered so many people along Bay Area nature trails that some parks posted ominous "Do Not Hike Alone" warning signs.

An Oregon family of five vanished days before Christmas. Was it an accident or murder?

Catherine Parr was the last of King Henry VIII's six wives and the only one to neither be cast aside nor killed by the volatile English monarch.

During World War I, German saboteurs ignited 2 million pounds of munitions on Black Tom Island in New York Harbor, triggering one of the largest explosions to ever occur on American soil.