Talk about it here.
The Sephiroth was a spaceship used by the last of the Jagaroth to reach Earth 400 million years BC, when the planet was barren and dead. The Fourth Doctor believed that it landed in what would become the middle of the Atlantic Ocean. The ship was spherical, with three legs as landing gear. It had warp drive and secondary engines and was piloted by the Jagaroth Scaroth from the warp control cabin.
Scaroth was to use warp thrust to take off, but at warp power three, which he knew was suicide without the ship's secondary engines functioning. When it took off, it entered a warp field and was ripped apart.
The explosion of the Jagaroth ship splintered Scaroth into twelve different selves which were scattered across time, killed the rest of the Jagaroth and caused the radioactive explosion needed to start life on Earth. (TV: City of Death)