I went cenote diving two Aprils ago. I had about 85 dives at the time and I was OW certified. It was my first time diving, and it was just me and the DM. He did a superb job of explaining everything-the 1/3 gas plan, staying on the line, and following the triangles. We had not seen anyone in the first two cenotes. As we dove our way to the third, we passed a group of five with cameras leaving. We then dove into a room area which was completely suited out with no visibility AT ALL!!! I completely panicked for 30 seconds. I then remembered: 1) my first DM said the first rule of diving is NEVER PANIC, 2) I had plenty of air, 3) the DM was no more than 3- 4 feet ahead of me, 4) the line is within 2 feet of me, and 5) I can follow the triangles out if I need to. I then calmed down, we continued slowly swimming out of there and the visibility cleared. It was fine on the way back. After the dive, the DM said the other DM shouldn't have let his divers silt up the area, and he should report him, but he wasn't going to. And that's the only time I've had full blown panic.