Hi A_poster,
The falling man you see in the intro is Atrus that tries to escape from another Age. He links through the Myst Linking Book, thus disappears, and then the Myst Book continues to fall into the Star Fissure. The Star Fissure turns out to be some kind of portal to Earth. Even Atrus didn't know that the Star Fissure led to Earth and where his Book went, that's what he tells you in the intro. So the Book lands somewhere on earth and is then found by the Stranger, the one you play in Myst. So the Stranger is not at the bottom of the fissure, but somewhere on Earth.
I hope that clarifies things.
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