In the notes for a print on Slowly Downward, Stanley Donwood writes:
In a book called “Brought to Light” by Alan Moore and Bill Sienkiewicz the numbers of people killed by CIA-sponsored state terrorism is measured in swimming pools. The average human body holds a gallon of blood. An Olympic swimming pool holds fifty gallons. The pools mount up through the book’s narrative, reaching a number I no longer wish to recall. This image haunted me throughout Radiohead’s Kid A project, and here I’ve coupled it with a scary bear; the record’s iconic image, and a symbol of looming danger and shattered expectations.
The mentioned book, Brought to Light, was published in 1989. The swimming pool image Donwood references appears in the artwork for Kid A as well as in this antivideo.