Tokyo mystery balls
One of the mysteries of the world, and of Japan, are these almost perfectly round stone balls that are usually found underground. Are they from space? Are they artifacts of an ancient, superior civilization that used to occupy these islands but has now been forgotten? If that's true, what could they have been used for? Some kind of ancient computer? Or time-keeping device? Or astronomical device? Ships' ballast, or mercantile weights? A weapon, or part of a weapon? Or maybe they were ceremonial cultic things, used in spiritual rituals.
Periodically they are dug up from gardens, construction sites, from beaches, while tunneling in the mountains, etc. They are about 45 cm in diameter, and people often use them as garden decorations, as curios. In this case there are several of them used as curbside decorations on a street in Suginami Ward, Tokyo.