Reference:RAWA/The Star Fissure and the Stranger

Q: Who am I in Riven?

A: The story of Atrus' life tells us that someone from Earth stumbled onto a Linking Book to Myst. For the sake of Myst and Riven, that person was you. You found the Linking Book to Myst somewhere on Earth; and the rest, as they say, is history.

Q: What is the fissure?

A: Here's a snip of info about the fissure:

The fissure remains one of the most inexplicable elements in the story of the life of Atrus. It is known that it came into being when Anna and Catherine made their changes to the Descriptive Book of Riven (see the Book of Atrus), however, the cracks which they wrote were filled with lava.

Theories persist about the creation of the star fissure being caused by Catherine's bizarre writing style, or perhaps a contradiction in her additions to the Descriptive Book. However, the fact that the fissure acts as a gateway specifically back to Earth seems to imply that the fissure wasn't created by random chance, and that the Maker himself created the fissure to suit his own plans.

This is the conclusion that Atrus and Catherine came to, and is briefly explained by Catherine in her accounts of Atrus' life.

Q: The rendering of looking up at the gaping hole of the fissure looked exactly the same. Did you guys use the original rendering?

A: No, this was remodeled in Softimage, but it matches the Strata model in every important respect.