After leaving the Void Between Worlds, the Greth arrived on Earth in "the Infernal Depths" beneath the sea. It had been helped in this by a "patron" it met in the Void, (PROSE: Out of the Box) the Yssgaroth; (PROSE: Preternatural Nights) some accounts suggested that a portal to the Yssgaroth's realm existed at the centre of the Earth. (PROSE: Interference, The Book of the War)
Tardis