Paradox machine

The Paradox machine was a type of technology that was capable of creating a tear in space and time, and looping history back on itself. The Master built such a machine by cannibalising the the Doctor's TARDIS. The device created a tear in the sky, allowing millions of sphere-like Toclafane to attack Earth and its populace. The Master himself considered it to be his "masterpiece". (DW: The Sound of Drums)

Though the TARDIS was heavily modified by the Master, it still managed to ring its cloister bell as a sign of distress. Martha Jones also observed that it appeared to be "sick". It is not precisely known when the Master transformed the TARDIS, but it presumably occurred sometime after he took Lucy Saxon to the end of the universe. Once he became Prime Minister, the Master no longer required the TARDIS's time travel capability.

The paradox machine was designed to prevent a time paradox from occurring when the Toclafane were introduced onto Earth. Since the Toclafane were Humans from the future, any attacks on their ancestors would have prevented their existence, therby erasing themselves from history. However, the use of the paradox machine prevented such an outcome, allowing the Toclafane to enslave the Earth's population. The paradox machine was destroyed by Captain Jack Harkness, who opened fire on it with a machine gun. The destruction of the machine reversed time to the point it was activated, undoing everything that had occurred since then. (DW: Last of the Time Lords)

The interior of the paradox machine was lit by an ominous red glow. This is presumably an indication of the TARDIS's ill health, as the dying TARDIS in Turn Left had a similar red lighting. The main console in particular had gone under many modifications by the Master: The console itself was enclosed inside a metal cage, with various tubes and appendages attached to it. At its base was a pipe connecting to another area of the TARDIS, with a dial showing a readout of the pressure build-up inside. It is unknown exactly how much of the TARDIS was cannibalised or whether any other rooms were lost or ejected. However, the paradox machine appeared to somehow use steam power, as it shot out puffs of steam when it was activated. Jack destroyed the machine with a machine gun causing time to reverse to the moment the machine was activated. In the new timeline the machine never activated and time was changed.

''As the TARDIS was capable of preventing a paradox as long as the people creating the paradox stayed inside, the machine may have simply increased this power so that it could be used outside of the TARDIS.