Unable to fully comprehend the beauty and magnitude of what is now known as Megadome Cave, I captured one more image in my mind’s eye before exiting the massive underground chamber. The incredible soaring dome was one of the most beautiful sights I had experienced in a cave. At once, I was sad to be leaving such a stunning vista, but anxious to share the discovery with the rest of my team.

With mixed emotions, I slowly began to ascend from 180 fsw through the restrictive, silted-out chimney. About twenty feet from the cave’s terminus at the seafloor, the top of my head bumped into something soft, pliable, and obviously very alive. I immediately assumed that the huge goliath grouper I had backed down the chimney during my descent — possibly the same fish from my first dive several weeks ago — was now standing his ground above me, blocking my only exit. Anxiety flowed and ebbed as I considered several less-than-pleasant scenarios. How long would it take to cut my way through a 300-pound fish with my Z-knife’s one-inch blade?

Switching to my left to O.K. the guideline I had laid twenty minutes earlier, I maneuvered my halogen light above my head to view the obstruction through the silty darkness. Although visibility was next to zero, a wave of relief washed over me when I saw the light tan underbelly of a nurse shark inches above my mask. The four-footer was thrashing about and confused, unable to uncoil his body in the narrow tube we both now shared. After half a minute of pushing up on his sandpaper beanbag stomach, he finally extended his body and swam down and past me.

Relieved, I continued my ascent to the seafloor with no further obstructions. The dim glow of sunlight became visible only in the last few feet of the chimney, masked by the silty remnants of the hurried movements of large fish and my exhaust bubbles. After exiting the cave at 130 fsw, Rusty Farst and I clipped on our deco gas and began our slow journey back to the surface. The excitement of the discovery — and a school of almost two dozen blacktip sharks — made the lengthy decompression fly by. As Eric Reintsema and Al Barefoot made their way past us down the anchor line, I realized how inadequate sign language really is. Divers desperately need a hand sign for “un-freaking believable.”

Lightning can definitely strike twice. Megadome Cave is the second deep Gulf of Mexico cave E.U.G. (Explorers Under the Gulf) divers have discovered off Sarasota, Florida, in less than eight months. E.U.G. divers discovered the first cave, Diamond Rock Cave, in September 2003 (see Advanced Diver Magazine, Issue 16/2004). These won’t be the last caves found on the continental shelf off Florida. An ever-increasing number of divers breathing trimix at depth will undoubtedly result in additional deep cave discoveries.

In late April of this year, Jim Culter (an E.U.G. diver and scientist at Mote Marine Laboratory) and I set out from Sarasota, Florida, to rendezvous with Eric Reintsema’s boat 60 miles offshore at Diamond Rock Cave. Al Barefoot had provided Jim and me with a newly acquired set of GPS coordinates to check out on our way offshore, as our course would have us passing nearby.

GPS coordinates are not set in stone; there can be variations from one machine to the next. The weather was awesome — sunny, light winds, and a 2–3’ swell from the southeast — just what we needed to refine the numbers we had been given. Approximately 40 miles west of Sarasota, we zeroed out the GPS over a barren, perfectly featureless section of Gulf bottom.

After 30 minutes of running a spiral search pattern, we finally marked a spot of interest on the depth finder and noted the new numbers. The feature was not too exciting — just an extremely small depression with limited relief on an otherwise flat seafloor. In retrospect, it’s hard to believe that this inconspicuous blip on the depth finder would turn out to be the entrance to yet another undiscovered cave system, now known as Megadome Cave. Jim and I agreed that, given enough time later in the day, we would come back and dive the spot — just in case.

Less than an hour later our boats met as scheduled 20 miles further offshore at Diamond Rock Cave. The morphology of the system is that of a restrictive chimney starting in the middle of a depression in the seafloor at 180 fsw and emptying into a large chamber at 240 fsw. The room is 50 feet in diameter at the top, extends to about 400 fsw, and is somewhat hourglass shaped. Our dives were spectacular as usual. We were able to collect water samples inside and outside of the cave. Jim strapped a Hydrolab water-sampling device to my doubles so that temperature and salinity would be monitored continuously throughout my dive.

By the time everyone had completed their decompression obligations, boarded the boats, and stowed their gear, it was getting late. Fortunately, the previously nice conditions were now even better; the Gulf was becoming as smooth as glass. Jim and I were ready to leave first, so we relayed the new improved numbers to Eric and Al, and began our 20-mile run east to the mystery spot.

After anchoring, we readied our gear, then waited — and waited — and waited. Over an hour went by and no sign of the other boat. No radio contact, nothing. Now the sun was low on the horizon and the breeze was beginning to cool. We debated the option of taking the easy way out and just heading home. But this feature had a steep funnel shape that beckoned a closer look. Curiosity prevailed, and we decided to dive.

A strong surface current gave way to a school of friendly amberjack — the ambassadors of Gulf bottom structure — as we neared the anchor. The rocky entrance to Megadome Cave is located at the bottom of a sandy funnel in the seafloor at 130 fsw. The rocky outcropping at the base of the funnel is small, less than ten feet in diameter, covered in encrusting growth, and teeming with fish. In the fading light, the scene reminded me of a mountain valley bordered by snow-draped peaks. I knew at once that we had hit pay dirt again. If not for a cave to swallow the shifting sands of the seafloor, this depression would fill up overnight.

As Jim and I approached the feature, a large goliath grouper swam into the funnel and disappeared from sight under a small ledge. Shining my light into the darkness under the rock, I was thrilled to see a solution shaft just past a small duck-under. The configuration required a vertical headfirst entry. Alternately scraping my belly, then doubles, I slowly made my way down and into the cave.

My light did little to illuminate the situation; visibility had been reduced to only a few feet. During his hasty retreat, the goliath grouper had disturbed the fine sediment that covered the chimney walls. Arms outstretched in front of me, I slowly picked my way inch by inch down the silty tunnel. Repeatedly drumming his rage at my intrusion, he was somewhere below me, concealed by the silt.

Goliath grouper (Epinephelus itajara) are abundant in the Gulf waters of southwest Florida. They are the largest members of the grouper family, growing to an incredible 800 pounds. Anywhere structure punctures the otherwise barren seafloor, they flourish. Known for their immense size and fearlessness when harassing spearfishermen, goliath grouper are not reticent with their emotions. When upset, the large fish forcibly slam together the walls of their air bladders, causing a concussion in the surrounding water that produces a loud, low-frequency noise known as “drumming.” Unfortunately, cave entrances provide the perfect hiding spot for these bulky fish. For divers, they present an additional safety concern not encountered in landlocked caves.

I had no way of knowing if this shaft bottomed out or opened into a large room like at Diamond Rock Cave. I was concerned that if the tube did end shortly, the huge fish might “flounder” in the bottom, nervously awaiting my approach. I hoped that he would stay deep and not make a run up and past me toward the freedom of the cave’s entrance at the seafloor. There wasn’t enough room in the tight tunnel for a diver to turn around, not to mention for both of us to pass each other.

Since those first two dives, much more data about the Megadome Cave system has been recovered. The only entry into the system, the chimney referenced above, intersects one side of a massive underground domed chamber at 175 fsw. The chamber itself is oblong, almost football-shaped. The dimensions at 175 fsw are 85 feet in length by 65 feet in width. The top of the dome rises to 150 fsw, leaving a scant 30 feet of rock between the top of the dome and the seafloor above.

Al Barefoot, Curt Bowen, and Eric Reintsema surveyed the depths immediately below the entrance into the dome room. Below the chimney, at 310 fsw, is the top of a debris mound formed from the infilling of Gulf seafloor sand, silt, and other detritus over thousands of years. This pile of sediment slopes off to a depth of 340 fsw below the near wall. As of this writing, we have only surveyed the far wall to a depth of 350 fsw.

Encrusting yellow and orange sponges and sea urchins in the upper reaches of the dome are evidence of tidal pumping of nutrient-rich Gulf waters. Colonies of tubeworms form hard, sponge-shaped structures infilling solution tubes in the ceiling of the dome. Jim Culter and other E.U.G. divers are actively engaged in scientific sampling at both Megadome Cave and Diamond Rock Cave.

The Megadome and Diamond Rock cave systems have much in common. The entries to both systems are through restrictive chimneys that empty into large underground chambers. These features formed when sea level was much lower than it is today. Over many thousands of years, slightly acidic groundwater eroded the limestone bedrock it flowed through and produced the huge underground voids.

Given more time above sea level, the ceilings of these structures would have collapsed and formed sinkholes. Once sea level rose and flooded them, these sinkholes became the blue holes divers are familiar with. Offshore of southwest Florida there are quite a few blue holes — Captiva Blue Hole, Amberjack Hole, Green Banana, and others. Hopefully, in the not-too-distant future, we will know much more about these beautiful and exciting karst features.