A man police said was the "librarian" of a global internet child abuse ring has been jailed indefinitely.
Philip Anthony Thompson, of Stockton-on-Tees, will serve at least three years and nine months, but a Teesside judge warned it could be much longer.
Thompson, 27, admitted 27 charges, including causing children aged under 13 to engage in sexual activity.
The website he was involved with used "borderline-legal" images to bring together "like-minded" people.
Police have so far pinpointed 360 suspects.
Handing down sentence at Teesside Crown Court, Judge Michael Taylor said Thompson posed a "very significant risk" to the public and therefore he would be imposing an indeterminate prison sentence for public protection.
He told him: "You have shown that you are a very dangerous individual indeed. I consider that you pose a very significant risk to the public and you are a dangerous offender."
"He [Thompson] was... the librarian/warehouseman for a myriad of images that were distributed to like-minded individuals both in this country and elsewhere"About 241,000 child abuse images - which were still and moving images of varying degrees of extremity - were discovered in his possession.
Police said more than 130 suspects had been uncovered in the UK and more than 50 arrests have been made.
As a result, 15 children in the UK have been removed from situations where they were being abused or were accessible to an offender.
Police revealed the network was first infiltrated by Metropolitan Police officers, working undercover, in May 2007
It became the largest investigation of its kind in the UK, also involving Cleveland Police and the Child Exploitation and Online Protection Centre (CEOP).
Users of the forum would post borderline-legal images of children and pass comments on them.
Police said "indicative images" rather than explicit child sex abuse images were posted online in an attempt to keep the site "below the radar" and prevent it being shut down.
According to CEOP, which co-ordinates covert investigations into internet child sexual abuse, users would make contact this way then meet other like-minded individuals and exchange images in "different online environments".
Its chief executive Jim Gamble said: "This website - whilst appearing to operate on the margins of legality - was clearly a front for the sinister, sexual abuse of children and an image trading ground for paedophiles."
Det Chief Supt Mark Braithwaite, from Cleveland Police, said Thompson had been "a critical piece of this network".
"He was the librarian/warehouseman for a myriad of images that were distributed to like-minded individuals both in this country and elsewhere."
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