Christ Goldman

And why would the Bow Tie Nebula be the coolest place in the universe, I wonder?

The Racnoss evolved in the Dark Times of the universe. (DW: The Infinite Quest) They were nearly wiped out by the Fledgling Empires over 4.6 billion years ago. Nearly all of the survivors of the race escaped in their spacecraft, around which the planet Earth would later form.

At the height of their civilisation, the Silurians lived in cities of metal millions of years old, and had uncontested dominion over the planet. However, this era was cut short. Some of their scientists, such as Doctor Wolik, believed that a small planetoid was destined to crash into their world. Some Silurians doubted this coming disaster; a few believed it to be a simple scare story. Eventually, the Silurians panicked. They entered hibernation chambers so their civilisation could survive the coming disaster and reclaim their world once the danger had passed. (DWM:Twilight of the Silurians)

Twelve years before the large planet came dangerously close to Earth, the Silurians were divided on the matter of a mating between the land castes and aquatic castes. Normally, such matings were forbidden, and any offspring were killed along with their parents. However, facing the threat of a global cataclysm, they allowed a colony of such hybrids to remain in their Shelter near Smallmarshes. With the rest of the Silurian race, they took part in theGreat Hibernation. (MA: The Scales of Injustice) The panicked Silurians sealed themselves in their undergroundhibernation chambers, but the technology was faulty, and never woke them. They were unaware the dangerous planetoid that they believed would devastate their world was instead captured by the planet and became the Moon. (DW: Doctor Who and the Silurians) Despite their intentions to punish Tulok, the renegade Silurian scientist instead sabotaged a number of the hibernation chambers and destroyed them. (BFA: Bloodtide)

The Silents remained hidden on Earth for hundreds of thousands of years, since the early Stone Age. They had the unique ability to be “memory-proof”; anyone who saw them would instantly forget about them the moment they looked away - even if they saw a photograph or recording of the Silence. Anyone looking at the Silents would recover their memories of previous sightings, although not instantaneously. The Silents were also impervious to at least some types of scanning devices, as River Song’s bio scanner couldn’t detect them in the underground tunnels. (DW: The Impossible Astronaut)

(via, links and text from TARDIS Wiki)