Screens of Zolfa-Thura
From Tardis Wiki, the free Doctor Who reference
The Screens of Zolfa-Thura were large metal plates situated on the barren planet, Zolfa-Thura. There were four plates in total that could amplify signals. Meglos made use of a room hidden in the centre of the plates. This room could rise up during the amplification process.
Meglos installed the Dodecahedron atop the room and intended to use amplification to destroy Tigella. The Fourth Doctor reverted the calculation and it destroyed Zolfa-Thura.
Prior to this, the Doctor and Romana II knew of the plates and their power. (TV: Meglos)
