The problem with ninjas is that they're very quiet. They sneak into your castle at night, evading guards, slitting people's throats and then disappearing into one of those annoying smoke bomb clouds that take forever to air out.

A rare, 17th-Century woodblock-carved ninja selfie

In 16th-Century Japan, rumors of ninja prowess began to spread. And by the 17th Century, clever castle-builders designed a countermeasure to alert sleeping residents if there was a ninja sneaking around.

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The design feature was called uguisubari, or "Nightingale floors." When one walked over the floorboards, this precise noise was produced:

The builders achieved this bird-like noise by placing metal against metal. Beneath the joists, they mounted extraneous metal brackets with nails driven through them. As the floorboard is stepped on, the brackets flex, producing the noise. These diagrams give you a not-entirely accurate idea of how it works:

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I say the diagrams are not entirely accurate because they give you the impression that the brackets move up and down, and that the mounting nails are the ones producing the noise. That doesn't make sense. If you look at the photo below, you'll see the mounting holes are not slotted, and the brackets are nailed in two places at an angle. This means the brackets cannot possibly slide up and down.

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And if you look closer at the photo, you can see telltale nail holes (arrows) indicating this bracket has been repositioned (probably because over time the wear of metal on metal stopped producing the noise). And we can see two bent nails (circled) left behind in the wood:

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Those bent nails are not accidents; Japanese carpenters are not known for sloppiness. If we imagine the brackets in their original positions over the old nail holes, we can see that they would clearly overlap with the bent nails. I believe the brackets were designed to have some flex in them, causing them to rub against the bent nails concealed behind them, thus producing the noise.

In any case, Core77 has managed to obtain some exciting security cam footage of an assassination being thwarted by the flooring. In this clip we see a female ninja hellbent on murder—but halfway down the hallway, she realizes she's making too much noise and abandons her mission: