Woah... living quicksand, so good at sucking you in that the only chance of escape is to
sever the part of the body that is in contact with it. And anyone swallowed by it, their souls stay in there are and in constant agony?! Even a century later?! That is some dark stuff!
I was about to try and write an explanation, then realised I could just rip the explanation from wikipedia:
https://en.wikipedia.org/wiki/SCP_Foundation"The SCP Foundation is a fictional organization that is the subject of a web-based collaborative writing project. (SCP stands for "Special Containment Procedures", though it also refers to the organization's slogan of "Secure, Contain, Protect".) The stories generated by the project describe the exploits of the fictional Foundation, supposedly responsible for containing individuals, entities, locations, and objects that violate natural law (referred to as SCPs). The majority of the written works on the SCP Foundation website consist of "special containment procedures", each of which purports to be the containment process for a specific SCP. The website also contains several hundred "Foundation Tales", which are short stories set within the universe of the SCP Foundation.
The SCP Foundation series has received praise for its ability to convey horror through its scientific and academic writing style. The SCP Foundation has also inspired numerous spin-off works, including the video game SCP – Containment Breach."