I take my hat off to the people who do that... It certainly takes a lot of guts.
Swimming along subterranean passageways with no idea where they will lead or how long they are.
I've swum? in the underground ones for quite some distance, but they all had a cave structure and cenote holes now and then.
Our friend actually swam under-water in very tight conditions and came up into another opening on the surface in a lagoon. It was scary as he was underwater for a long time. He could hold his breath for a long time in those days, don't know if he'd try that now.
This would be very bad for the Yucatan if all these underwater rivers were connected for all their sewerage dumps into them and far out into the ocean. Like they said in the article, the sewerage could eventually connect and all that gorgeous water could get contaminated.
Now that would be nasty. :(