(Photo: Zen Garden)
When you lie down to sleep in Edo period Japan, will you wake up in the morning? By the 17th Century, the threat of assassination by ninjas had become serious enough that architects in Japan developed a special alarm system to help aristocrats stay safe.
(Photo: Chris Gladis)
It's called the Nightengale Floor. Here's an example from Nijo Castle in Kyoto. The floorboards are attached to support beams with metal brackets that scrape against each other. When someone steps on the joint, the brackets make a sound like a nightingale chirping.
As this video demonstrates, it really does sound like a chirping bird. So if you hear this sound late at night, arm yourself.