Odin threw the serpent into the sea while it was still small enough to throw. It kept growing until it circled the world entirely and closed its jaws on its own tail.
Thor meets it more than once.
He goes fishing with the giant Hymir, baits a line with an ox head and hooks the serpent. He has it at the side of the boat with the hammer raised when Hymir works out exactly what is about to happen to the world, and cuts the line.
They meet again at Ragnarok. Thor kills the serpent, walks nine paces, and falls.
The ring in this design is unbroken. Every ending inside it is also a beginning, which is the part people tend to forget about Ragnarok.