I haven’t yet seen anything about whether this is basically standing pooled water, or is a genuine underground creek.
One diagram indicated there is another vein that meets up half way to their location, and THAT is where overflow comes from. So that entrance must be uphill, while the entrance they walked into must be basically level with them. And it implies to me that there is no “flow” per se - only as a consequence of current rains flowing in, which implies to me that that vein is essentially a run-off creek only, and not a genuine permanent creek. Then I’d think at least they don’t have much “current” to deal with, but some statements make me question that.
I think it’s an underground muddy stream that is subject to changes quickly, rainfall induced.