Appears In: Myst, Riven, Uru: Complete Chronicles, Uru Live
Location: Riven's Temple Island (Myst, Riven), Relto (Uru)
It should be noted that the Star Fissure is for some reason linked to the cleft. The wreckage of Gehn's telescope can be found there, along with some Wahrk bones (as seen in Uru). This fact seems clears up a lot of speculation about the Fissure and where the Stranger came from.
The Star Fissure is a small crack in the ground in Riven that is full of stars. It looks exactly like space, but it is actually hospitable to life. One of the things that makes the Fissure different than space are the large black areas where no stars can be seen. As the Fissure itself is located in one of these black areas, it seems safe to assume that these patches are other Ages. The exact nature of the Fissure is unclear from available writings; Atrus and Catherine made no real attempts to explain it, only saying that it was from the Maker himself.
Yeesha seems to have developed the ability to better control the appearance and stability of the Star Fissure in her Ages. Upon completing her Journey as part of Uru, a Fissure appears in Relto and directs you back to the Cleft. Unlike the Fissure in Riven, this one seems far more stable, and does not generate a vacuum-like effect within the Age. Also, unlike Riven, the Fissure in Relto seems to be able to close itself up again after it is used, leaving a stable Age behind.
There is evidence to suggest that the "blue" Bahro Cave is actually within the Fissure; Linking Books fail to work within it, and beneath it is an infinite expanse of stars. As explorers report being linked back to Relto without resorting to a panic-link, it would appear that, like the Fissure in Relto, the void-like realm beneath the blue Bahro Cave has linking capabilities.