He fell into mako. It travels through the planet like the lifestream, if the two are one and the same. (SPOILER)When Cloud fell into the Mako in North Crater and washed up in Mideel, it was the same concept. They just kind of floated around until they got to their destination. It's actually likely that Sephiroth ended up where he did. The Northern Crater was fertile with mako, so if he's going to flow through mako, why not go to where it's most abundant?