The Ariake Incineration Plant is an easily recognizable building in the popular bayside area of Odaiba, with a smokestack standing tall at 460 feet (140 meters). While it’s not much of a landmark or a tourist spot, it’s fairly popular among the locals for the adjacent Sports Center, whose power is sourced from the plant.

On one side of the big chimney is a peculiar symbol composed of dots, just a circle, and a vertical line, which almost seems to be lit up at random. Since its completion in 1995, locals and visitors alike have long been baffled about the nature of this apparatus. As it turns out, finally confirmed on the Incineration Plant’s website in 2014, it is in fact, supposed to be a clock.

To get the time, one has to first check the circle, which has 12 dots. Easily enough, it corresponds to the hour; if five of the dots are lit, for instance, it simply means that it’s five. Now, the minute is the tricky part. It is represented by the vertical line, which also has 12 dots that light up one by one, every five minutes. So, if three of these dots are lit, it means that it’s between 10 and 15 minutes.