Vashta Nerada Vashta Nerada , literally , " the shadow s that melt the flesh " , were swarming , carnivorous beings which were found on most planets . The Doctor described them as the piranha s of the air . ( AUDIO : " Night of the Vashta Nerada " ; TV : " Silence in the Library " ) Biology . The Vashta Nerada were microscopic beings that lived in swarms , thousands strong . Individually , Vashta Nerada were not a threat . They were very small ; the Tenth Doctor claimed that some of the dust specks visible in bright sunlight were single Vashta Nerada or small swarms , not large enough to be a threat to most life . In large numbers , they could strip a creature to its bare bones in milliseconds . ( TV : " Silence in the Library " ) In extreme cases , they could tear apart the stone bodies of silicon - based lifeforms , though it was not necessarily edible to them . ( AUDIO : " Night of the Vashta Nerada " ) Vashta Nerada swarms were sentient and two spacesuit - inhabiting swarms developed the ability to communicate with the Doctor through the spacesuits ' neural relay s . ( TV : " Forest of the Dead " ) They also coordinated their efforts and made the spacesuits walk , though with some difficulty ; they walked slowly in a " zombie - like " fashion . The Vashta Nerada lived in the darkness . They cast shadows when they entered the light — even stopping the reach of light from fission lamp s . On their own , outside of the darkness , a Vashta Nerada swarm looked like a shadow cast by nothing . They mimicked the shadows of their prey to get close , which meant staying in the light was the only way to escape . Light also slowed them down somewhat , but if the swarms were great enough , the effects of light would be minimised and overcome quickly . If someone had an extra shadow , it was already too late . The Doctor could detect the difference between a Vashta Nerada swarm and an ordinary shadow by using his sonic screwdriver . ( TV : " Silence in the Library " / " Forest of the Dead " ) The Doctor believed they had no weaknesses , and the only thing to do was to run . ( TV : " Silence in the Library " ) Besides the swarm in the Library , the Doctor indicated that the Vashta Nerada were generally harmless . Existing on over a billion planets including Earth , the Vashta Nerada inspired a fear of the darkness but were nowhere near as aggressive as they were in the Library . The Doctor stated that he ' d never seen a swarm as large or as aggressive as the Library swarm . However , this unusual aggressiveness came from the fact that the books were printed with paper from the trees of the Vashta Nerada ' s forests and thus they saw the Library as their " forest . " ( TV : " Silence in the Library " / " Forest of the Dead " ) Vashta Nerada engineered on Synthesis Station possessed the ability to consume Dalekanium , the Time Lord s who supported the project wishing to use them against the Dalek s in the Last Great Time War . A negative strain of Vashta Nerada that thrived in the light were also created , named Nerada Vashta . ( AUDIO : " Day of the Vashta Nerada " ) Life cycle . Earth and " a billion other worlds " , such as Funworld ( AUDIO : " Night of the Vashta Nerada " ) and the Library , had Vashta Nerada living there . The Tenth Doctor told Donna Noble that " Where there ' s meat , there ' s Vashta Nerada . " ( TV : " Silence in the Library " ) They were born as microscopic spores in trees and lived their lives in forest s . These spores were resilient enough to hatch even if the tree was converted into paper . ( TV : " Forest of the Dead " ) History . The Tenth Doctor and Rose Tyler encountered the Vashta Nerada at the Lake Widermere . ( COMIC : " Untitled " ) While Vashta Nerada swarms were not common , they inspired fear of the dark in most species , since any shadow could be made of Vashta Nerada . According to the Tenth Doctor , this was why almost every species in the universe was afraid of the dark . They knew , at the back of their minds , that the Vashta Nerada were there . ( TV : " Silence in the Library " ) On Earth , however , the Vashta Nerada were largely harmless and benign . ( GAME : " Shadows of the Vashta Nerada " ) They survived by scavenging road kill , but the Tenth Doctor mentioned that some people who walked in the dark never came out . ( TV : " Silence in the Library " ) The Vashta Nerada attacked a Russian space shuttle in 1965 , but were stopped by the Eleventh Doctor . ( COMIC : " Space Oddity " ) In the 23rd century , a temporal rift resulted in hostile Vashta Nerada arriving in the underwater city Poseidon 8 on Earth , along with a giant shark - like being . The Eleventh Doctor and Amy defeated some of the Vashta Nerada by luring the diving suits they were using into a brightly - illuminated area ; the Doctor later closed the rift and returned the Vashta Nerada to their original planet . ( GAME : " Shadows of the Vashta Nerada " ) In the 50th century , one planet suffered a gigantic infestation . The Vashta Nerada usually lived in their forests , but when some of the trees were cut down and made into books to be put in the Library , some of the spores were brought along . Over a " million million " Vashta Nerada hatched from the books , ( TV : " Forest of the Dead " ) so many that when the Library ' s life form scanners were later used to identify their presence individually , the scanners hit their capacity of 1 trillion life forms , the maximum number they could record at any given time . ( TV : " Silence in the Library " ) They regarded the Library as their " forests " and its visitors as their " meat " . When the Vashta Nerada hit their hatching cycle , the swarms infested the entire library . In response to this , the Library ' s data core , CAL , teleported everyone inside its system but could not send them back out because nowhere in the library was a safe place for them to make an escape . The 4 , 022 people on the planet could not be evacuated and were " saved " in its computer ' s hard drive . Because it was impossible to ever send the people back to their physical lives , the computer created a virtual world for them to live in which hid the truth from them for their own good , ( TV : " Forest of the Dead " ) and CAL sealed the planet off in quarantine . The Library , save for its Vashta Nerada horde , was left abandoned and empty for a hundred years until an archaeological expedition which invited the Tenth Doctor and Donna Noble landed on the planet . ( TV : " Silence in the Library " ) As their skills grew , they displayed better control over their victims as well as craftiness and savagery . Whole swarms working in tandem could chew through the mesh of the archaeological team ' s spacesuits with ease when its fibre density was not amplified , puppeteer the skeletal remains of those they had eaten to attack others , and envelop their soon - to - be eaten prey in a disorienting veil of darkness before killing them . ( TV : " Silence in the Library " ) At their most ruthless , the Vashta Nerada even masqueraded as a human being - Anita , who had tried to confuse them by letting her spacesuit ' s visor be completely darkened so they would think they already entered her suit and possibly leave her alone . However , because Anita ' s face was now unobservable , the creatures instead used this disguise to their advantage and ate her unnoticed beyond the second shadow they cast from her body disappearing . The swarm then used her data ghost ' s voice in the neural relay and began speaking as though they were her , but were quickly discovered as Anita ' s murderers when it was evident the second shadow they cast from her body had disappeared . Though much of the archaeological team was eaten by the Vashta Nerada , the Doctor convinced the Vashta Nerada to stop and give him a chance to return the people stored in the Library ' s computer . While the Doctor had no way of defeating them , he knew the Library would hold information about him and had the Vashta Nerada look him up . His fearsome reputation alone was enough to make the Vashta Nerada back down for a while . They agreed to give him a day after reading about him in the Library . However , CAL did not have enough memory space left to transfer the 4 , 022 people inside her back to reality , and the only way to compensate was for a living subject to let her borrow their memory space . The Doctor tried to donate his , but Professor River Song forcefully took his place , aware that whoever did this would certainly perish . Out of her sacrifice , the humans were rescued and left the planet to the Vashta Nerada , while she and the other lost members of the archaeological team had their data ghost s uploaded by CAL and given new life in her virtual world , redesigned into a peaceful paradise that would accommodate them . ( TV : " Forest of the Dead " )