Ley lines were cracks and bubbles of unimaginable levels of psi energy. They were one of the last surviving magicks of the universe that had not been cast out by the Gallifreyans when they became the Time Lords. This was because they were the least impossible sort of magic and the closest to actual science. A residue of psi energy formed a network of ley lines which stretched forth across the galaxy in improbable directions.
The Time Lords had long been aware of ley lines in times that predated the various Wars that they had fought. As they attempted to discard irrational magick in favour of rational science, a number of citizens objected; so the Time Lords turned the psi lines into weapons. One such waypoint of the ley lines was the planet Iphigenia that served as the basis for the Time Lords' Distant Early Warning line. This stretched through the galaxy and was studded with various receivers that were the size of mountains or small moons. Great parabolic dishes, disguised as craters, monitored the ley lines for eruptions of psi energy from beyond Gallifrey. (PROSE: So Vile a Sin)