I spotted this whopper in the window of a fancy fruit parlor right by the crossing in Shibuya. It's a Taiyo Suika — "Sun Watermelon" — highly appropriate for this time of year.
It's the size of a basketball and pretty much the same colour. Inside, though, they look quite normal, red and juicy just the way they should be. Apparently, they are extra sweet, though I've never actually tried one. For one thing, they're pretty unusual; for another, at ¥5,250 apiece they're not cheap...
Although compared to one of these whoppers (below) they are. Shikaku ("square") watermelons are status symbols, intended as prestige offerings for the mid-summer gift-giving season. Hence the ribbon on top. And the ¥12.600 price tag.
To produce these cuboid fruit, they have to be raised, handled and packed with extreme care. One advantage: their shape make them much more convenient for fitting in a box and transporting. And for fitting in your fridge.