Exploring Ursus

Exploring Ursus

I had my hand circled around a thin, twisted strand of cave line, attempting to stay still and neutrally buoyant while muddy microbial goop rained around me. The visibility in the cave wasn't zero – I could see my hand and my Teric, but the cave walls were invisible. The exploration line in my hand was all that connected me to Vince in front and our exit behind, and I could only hope that the visibility was better a meter ahead where Vince was laying line. Once again, we were cave exploring by braille.

Terrifying? A situation like this certainly could be, but in this case, I was not afraid. I made a deal with myself a long time ago that if I ever felt frightened or stressed during a cave dive, I would turn around and leave. When I am exploring, I am constantly assessing my mental and physical state. Am I clear-headed? Am I too excited, or am I cold and deliberate? How is my breathing and heart rate? How is my control? In this case I was calm, curious, and confident in my ability to exit, so I smiled and waited to feel Vince move ahead.

Exploring like this takes teamwork and over a decade of experience exploring together. I wouldn't do a dive like this alone, and I wouldn't do it with anyone other than Vince. We've explored and surveyed over 80 kilometres of cave together, and at this point, I can read his thoughts through his motions, and his mental state from the sound of his breathing. Sometimes I lay the line and sometimes he does, but we both know our roles. In this situation, my job was to check the stability of the tie-offs and the routing of the line, identifying issues before they occur, and making sure we have a diveable, unobstructed path on the way out – even in zero visibility. On a dive like this, the diver laying the line in front does his best, but if he can't see where he's going, he's dependent on the diver in the back to check the linework. Vince was doing well.

The cave was becoming deeper. I wouldn't have known this from sight, as the muddy goo blocking my view was disorienting, but I could still make out the glowing numbers from my computer and I was equalizing quite a bit. When we reached 24 meters, we broke through into the saltwater layer and the visibility abruptly cleared. It was almost spooky.

Vince and I found ourselves floating in the clear, still, blue saltwater layer, the higher fresher water layer churning and flowing above our heads. I felt like I had been pulled out of space and time and set aside into some alternate reality. It was silent and dark, and a massive tunnel opened in front of us. What looked like a filthy, clogged hole when we descended had turned out to be a giant. We grinned through our regulators and double-checked our gas in unison. What had felt like an eternity in the muck had only been ten minutes. This exploration dive was just getting started.


As with every other cave we have explored, Vince and I were not the original discoverers of the cenote. As always, the credit for finding the hole goes to a knowledgeable local, who was long aware of the entrance and kind enough share it with us. While L.E. prefers to remain unnamed and un-pictured, his expertise in the mangroves is what is truly responsible for our knowledge of the existence of this cave. Vince and I spent five years gaining the trust of locals in the region, before a chain of acquaintances and friends ultimately lead us to him.

Vince with the dingy


Vince named Cenote Ursus after the constellation, and the name means the bear. We came up with the name before we dived the cenote; it fits our regional naming scheme of mythological figures. We named the cave having no idea how fitting the name would turn out to be. Just like a bear, Cenote Ursus can be placid or violent, and at the beginning of the project we had no way to predict what the conditions would be like or whether we would be able to dive.

Now, with more experience (and several failed exploration trips), we are just beginning to be able to predict seasonally how the conditions in Ursus will be. However, even with a seasonal strategy, it seems that a variety of less predictable conditions also influence the conditions in Ursus. Sea level, which is related to weather, which is increasingly unpredictable, seems to play a role. Rain and the sargasso seaweed influx affect the amount of hydrogen sulfide in the water and therefore the visibility. The most concerning condition is the reversing currents. On several occasions, we have arrived at the cenote only to find it siphoning so strongly that diving would be extremely dangerous. Our next step will be to install a Cave Pearl flowmeter in the cave to monitor the conditions over an extended period.

Clear entrance with tannic stain


The first time Vince and I encountered microbial growth in caves was in 2014, on the day we discovered Cenote Pandora (now Sistema Pandora). We dropped into a tiny hole in the middle of mangroves to find meters-long, soft “stalactites” flowing gently in the water. We initially thought that these growths were unique, but we have now learned that they are typical of mangrove caves in which high concentrations of hydrogen sulfide are present.

Initial analysis of these soft strings of microbial life appears to indicate that it is a type of archea, a single-celled organism located way, way back on the tree of life. Final classification of the organism is still pending. Archea was first discovered in extreme environments, and while types of archea have subsequently been discovered in other, more normal habitats, it clearly thrives in hydrogen sulfide.

The location of Ursus in a mangrove forest, the quantity of hydrogen sulfide, and the soft, goopy sediment all seemed to indicate that microbial life would also exist in Ursus, but on the first few dives the visibility was so poor that we could not discern whether or not such growths were present. Months later, on clearer exploration dives further into the cave, we found small microbial chains hanging from the ceiling and while the strings do not appear to grow in the clearer salt water passages, we found fallen microbial growths on the floor in some of the passages that contain both fresh and salt water.

In Ursus, we have only noticed microbial strings thriving in areas where they would be somewhat protected from the water movement, and in approximately 10-centimeter growths along the ceiling. This is different from other mangrove caves we have explored, which often have meters-long growths. My hypothesis is that the microbes do not hang further down into the tunnel due to the strong, reversing flow conditions in Ursus.


The water in the video has a greenish tone, this is not an effect! The water in Cenote Ursus truly has a vivid emerald tone. The coloration comes from hydrogen sulfide (H2S), a toxic gas which is dissolved in the water. It's common to find hydrogen sulfide in mangrove caves, but this does not mean H2S should be taken lightly.

I know of no studies that have been published detailing the effects of prolonged H2S exposure on divers. However, breathing H2S gas in the surface can be fatal, even in relatively small parts per a million.

From firsthand experience, I can state that diving in light concentrations of H2S, while not fatal, does influence the diver. Skin exposed to H2S may become irritated, and heaven help the diver who accidentally gets water containing H2S into his mask or in his eyes. Anecdotally, I've heard that H2S can be absorbed through a diver's skin, making him ill and inhibiting decompression. While I don't have scientific data to back this up, I can say from my experience that multiple days of diving in light concentrations of H2S make a diver nauseated, cause headaches, and make a diver smell like rotten eggs when he sweats. For this reason, when exploring in even light concentrations of H2S, we use dry suits to limit our exposed skin and take a day off after a few days of diving.

The visibility reduction from hydrogen sulfide looks like a gray mist or fog. From a practical standpoint, the presence of H2S makes exploration more difficult as it inhibits visibility. In the freshwater portions of Ursus above 24 meters, we are slowly exploring along the cave walls to find passages that we would not have seen due to the reduced visibility. So far, we have found one promising lead, but there are certainly other tunnels hidden in the murk that we will discover as the project continues.

Interestingly, the density of H2S is such that it does not fall into the saltwater passageways below 24 meters. These passageways remain perfectly clear except for percolation created by our movement or bubbles. It's one of the reasons we have been focusing strongly on the deeper passageways in Ursus up to this point.


Ursus is remote, in time, in distance, and in some other, intangible sense. Travelling to Ursus feels like stepping into an alternate universe – not back in time, but into some sort of post-apocalyptic reality. Contemporary plastic trash litters the shoreline before we turn inland, evidence both of the presence of humanity and the lack of actual inhabitants to clean it up. It is silent and hot with a sort of churning energy filling the air (or maybe that's just the mosquitos). We are hundreds of kilometres away from tank fills and logistical support, there's no cell service, and if anything goes wrong, we are on our own.

Packed like Sardines in the dingy

We travel to the mangroves in a small dingy, piloted by a friend who's made it his business to know the ins and outs of the mangrove swamps in southern Mexico. When we first met him, his dingy was two-feet deep and floated about one inch above the water when loaded with Vince, me, and 4 tanks. We had to stay absolutely still while our friend polled us through the mangroves. The slightest motion would have capsized us and our gear into the shallow water.

Over the months, we have contributed what money we could to our friend's boating project, and he has carefully built up and reinforced the hull of the dingy. In the video, you see the dingy in its final form – much deeper and sturdier than the first day we carefully stepped into it. After the footage was shot, he was able to invest in a small outboard motor, and we are no longer required to tow the dingy through the swamps using a scooter (although that was fun).

The remote aspect of the cave is part of what makes it so much fun to explore. The difficulty and cost of organizing the logistics, travelling to the cenote, and finding that the conditions are unexpectedly adverse to diving can be frustrating, but it also makes the days when we can dive that much sweeter. Working together with our friend who owns the dingy in these situations has created a lifetime bond and strong friendship. Our dive team extends above water, branching into the friendship we have forged, and while he prefers not to be named, L.E. is as much a cave explorer as Vince and I are.


The deep section of Ursus appears to continue. Vince and I won't know if our end of line can be pushed until we dive again, but we are very hopeful. Our last dive in the 35-40 meter section of the cave left us gazing at a dark crack, but without sufficient gas to continue safely. Ripples in the floor indicate there is water movement through the passage on occasion, and if there is water movement, there is a passage. Whether or not it continues at a size that divers can fit into remains to be seen. But that's exploration – anything could happen and we wouldn't trade the alluring uncertainty of this mercurial cave for anything.