The Optical Illustration is a Ragú pasta sauce lid that is found in the Perplexing Pool in Pikmin 2. It's exclusive to the American version, and in the European version, the Abstract Masterpiece is found here instead. The text on the treasure reads:
Collecting the treasure
It is located near the Citadel of Spiders but is near an electric gate surrounded by water, making a volatile combination. First, defeat any Yellow Wollywogs and Shearwigs in the way. If the Dwarf Bulbears are there, take them out, too. Build the bridge near the stump found next to the Citadel of Spiders, then throw Yellow Pikmin over the wall to where the electric gate is. Alternatively, it is possible to throw Yellow Pikmin over the electric gate from across the water, with a running start for each throw; or a small number can be led along a root in the water on the left of the gate, then thrown over the wall easily. Let them tear it down and let Blue Pikmin carry it back to the ship.
“I've noticed that some of the objects we collect have undeniable similarities. Perhaps these treasures were revered by an ancient civilization.”
“There are many items that look like this one, but the contents are profoundly different. Those in the know are aware of the differences. This particular item should please any buyer.”
Names in other languages