Bristol Crown Court heard Christopher Daldorph, 45, spent up to two hours a day over 14 years viewing the material.
Officers from the National Crime Agency (NCA) found 20 devices at his home in West Wick, Weston-super-Mare, when he was arrested in April.
One hard drive alone contained more than 150,000 images and videos.
Daldorph admitted nine counts of making indecent photographs and one count of possessing extreme pornographic images between November 2010 and April this year.
Passing sentence, Judge Geoffrey Mercer QC, said: "For over 14 years, as you admit, you have regularly set time aside to scour the darkest corners of the internet to find some of the more disturbing representations of child abuse available.
"This is in my judgment an extreme case of downloading indecent photographs of children, because of the volume of the material which you downloaded over a substantial time."
Christian Hall of the NCA said: "Daldorph was responsible for amassing hundreds of thousands of indecent images, some of them the most extreme possible involving very young children.
"Each of those involved a child being abused in some of the worst ways imaginable."
Daldorph, who had worked for the NHS for 17 years, was dismissed from his job after he was arrested.
A hospital radiographer, who downloaded more than a million indecent images and videos of children, has been jailed for two years and four months.