What source does "suk" refer to? (an abbreviation cited in the "Jewish encyclopedia" as a source)

Sorry if this is a really basic question but I have no clue what that citation refers to…

I was reading an article in the “Jewish Encyclopedia” discussing an incident where King David reportedly “dug too deep” and accidentally opened an underwater sea that might flood the entire planet. He then reportedly stops the flood by throwing a tablet into the chasm… – I’m not familiar with that story and wanted to know more but the only reference I saw there pointed to “suk 531 b” which I’m not familiar with. (I see “suk” in a lot of other sources as well so it seems like it might be well known but something I’m not familiar with)