I was re-watching series 4's The Stolen Earth, and the Doctor mentioned: "someone tried to move the earth once before, a long time ago." Then it cuts to the Daleks, implying that it was they who attempted this. Does anyone know if there's a comic or episode about this? Or was this simply an offhand comment?
According to the Tardis Data Core/Doctor Who Wiki, this is a reference to the Second Season serial The Dalek Invasion of Earth, broadcast November-December 1964: The Doctor mentions that someone tried to move the Earth "a long time ago." The Daleks once tried to place a drive system into the centre of the Earth so that they could pilot the planet.
The wiki also notes that the Time Lords did actually move the Earth in the 23rd Season of the show, more commonly referred to as The Trial of a Time Lord: The Time Lords successfully relocated the Earth, causing it to become known as Ravolox.