1578
Sea Level
(imgs.xkcd.com)
Alt text: They're up there with coral islands, lightning, and caterpillars turning into butterflies.
I like to read science history books, the process of discovery and methods people come up with in search of answers is fun. Anyway, I read a whole book about tides, and one of the reasons people thought the tides rolled in and out was because somewhere in the ocean, a massive creature was slowly breathing in and out enormous volumes of water. These same people still went out on boats.