Heart of the TARDIS

The Heart of the TARDIS (or the Time Vortex) is the ship's soul. The connection to the Eye of Harmony is unclear.

When Blon Slitheen looked into the Heart she was reverted to a Raxicoricofallapatorian egg (Boom Town). When Rose did so she could telepathically operate the TARDIS and flew it to the GameStation (The Parting of the Ways). However, this power would kill her, so the Doctor sacrificed himself and took it himself.

It seems to take a great amount of strength to pull open the TARDIS console, revealing the Heart, such as a rift in space and time, or a large truck.