Rescue team searching for missing cave diver - Bulgaria

[Jack Hammer]

A siphon or sump is simply the term for any portion of a passage in an otherwise "dry" cave that is below the surface of the water. It has nothing to do with what direction the water is flowing, though sumps are probably encountered more often from upstream than from downstream. The only relevance of which direction the water flows is in terms of viz, since whatever silt you stir up will flow downstream.

I wasn't aware dry cavers referred to waters differently than cave divers do, thank you for clarifying that. In cave diving a syphon means water is flowing into the cave and requires special gas planning as it will take more gas and effort to exit the cave than it took to enter. Some siphons can be extremely dangerous and they will close access to caves while they are siphoning. Google pics of Little River in Florida when it's reversed as an example.

 

[fsardone]

I am a beginner dry caver and a cave diver with little experience.
In siphons or sumps very often there is no flow. The name (at least in my region) refers to a specific shape of the cavity where you have the opportunity for water to accumulate and close the passage.
Refer to
Sump (cave) - Wikipedia, the free encyclopedia
Often dry cavers empty sumps by siphoning
Siphon - Wikipedia, the free encyclopedia

You can have a sump with inward or outward flow (and we could start discussing what is what when you have a flow going underground and coming out somewhere else ...) but this has nothing to do with the sump shape of the cavity.
Usually if water goes underground it is called swallow hole and if water is coming out is called spring.
There is sometime confusion between swallow and sink holes and they are mixed up. But a sink hole involves a depression of the ground, maybe a cave ceiling collapse, while a swallow hole just allows water through and hence communication with permeable layers of underground. A sink hole maybe a spring or a swallow hole

Cheers
Fabio