The first time I saw this picture of square watermelons, I straightly shout in my mind "what a creative photoshop trick!" and I was definitely wrong. They are really square.
You probably think it's result by a genetic engineering but in fact those are made the old-fashioned way. They are grown in boxes, and take the cubic shape gradually as they grow. It’s a labor-intensive process, but the end result fits nicely in a refrigerator, and wastes no space in the truck. And they won’t roll around! They’ve been growing them in Japan for years, because space is at a premium. The watermelons are at premium prices, too.
To achieve such results, you have to have a proper box, made of tempered glass or durable plastic. Transparent boxes are best, or else you won’t know exactly when to harvest, or even worse, waste your equipment on a rotten fruit! K-mac Plastics sells boxes especially designed to grow watermelons in, complete with proper drainage.