Creature ( Thin Ice ) A sea creature , which resembled a giant fish , was imprisoned under the River Thames until 1814 . ( TV : " Thin Ice " ) Biology . The creature was massive in size , easily several times that of a whale . It shared many similarities with lure fish , namely the overall shape of its body , its fins and its bioluminescent lure on its forehead . The creature was female and could produce several offspring who were the size of regular lure fish . The creature ' s race were extremely long - lived and could potentially live for centuries , perhaps even millennia . The Twelfth Doctor speculated that they were of terrestrial origin , perhaps from Earth ' s prehistoric era like the Silurian s and the Sea Devil s . ( TV : " Thin Ice " ) Diet . The creature was carnivorous and fed on meat , mostly human s . ( TV : " Thin Ice " ) Abilities . The creature ' s presence alone seemed to cause intensely cold temperature s , enough to freeze the whole River Thames solid in winter . The creature ' s feces made for an ideal fossil fuel . It could burn slower and longer than coal , and could even burn underwater . ( TV : " Thin Ice " ) Behaviour . Neither the Doctor nor Bill knew what would happen when the creature was freed . Instead of lashing out at her captors , the creature , once freed , swam away to the ocean . Despite her colossal size , she was careful not to harm anyone or collide with the bridges and buildings lining the river at South Bank . ( TV : " Thin Ice " ) History . For an undetermined amount of time , the creature and her offspring were chain ed up under the River Thames by Sutcliffe ' s family . They somehow discovered that its faeces made for an ideal fossil fuel , which burned longer and hotter than coal , and decided to sell it to amass wealth and power . In order to ensure a constant supply , they would subtly influence the Frost Fairs that took place every year on the river Thames and have the offspring feed humans to their mother . ( TV : " Thin Ice " ) One account stated that Sutcliffe himself purchased the creature from the Master . ( PROSE : " Dismemberment " ) During the Frost Fair of 1814 , the Twelfth Doctor and Bill Potts arrived , the TARDIS having detected the creature ' s presence . Once they realised what was happening and that the creature was as much a victim as the people she was forced to eat , they freed her and defeated Sutcliffe . After she was freed , the creature and her offspring swam away to some northern location to live in peace . ( TV : " Thin Ice " ) Legacy . While newspaper ' s reported on the death of Sutcliffe , who apparently " drown [ ed ] in [ a ] snap thaw " , Bill Potts was later unable to find any results via search - wise . net for " London 1814 sea creature " . She also tried " serpent " and " really , really big fish " . The Doctor reminded her , " Never underestimate the collective human ability to overlook the inexplicable , " though also noted that the Frost Fair involved a lot of day drinking . ( TV : " Thin Ice " ) While floating in Titan ' s low gravity , Bill mentioned " sea monsters in the Thames " . ( COMIC : " The Soul Garden " ) When she moved into her new haunted house , Bill reassured herself that " there ' s no living puddles or weird robots , big fish . It ' s just a new house , and people you don ' t know . Not scary at all . " ( TV : " Knock Knock " )