In the Nezu district of Tokyo, there is a stone staircase infamous for its strange feature. Known as Obake Kaidan (“Ghost Stairs”), it has 40 steps on your way up—but 39 when you go down.

The staircase was once very narrow and dark, deserving of its nickname, but after an expansion work a handrail was added to it and the width of the stairs was doubled. Still, the staircase on the left trails off halfway for no reason, its steps are uneven, and a unique, mysterious atmosphere remains there.

Many urban legends surround the staircase, some plausible and some far-fetched, such as the steps being made of gravestones and counting them causing unaccountable deaths, but the truth is quite simple, albeit scarcely known.

If you’re being careful enough, you’ll notice that the first step on the bottom is extremely flat, almost level with the ground. This is why the step disappears: even if you count it when you climb the stairs, you’re likely to disregard it when it’s the last step.