From the picturesque little Dzilam de Bravo we took two boats up the coast. It was a very windy, sunny day and we all got a little wet from salt spray. However, with seagulls and big flocks of White Pelicans above us playing with the wind, the pleasing rhythm of the boat slapping into the waves, the stinging sunlight and the coolish breezes, we all felt more than content.
About half an hour east of Dzilam de Bravo, maybe 200 yards/meters from shore and in about two feet of water, we came upon a spot maybe five feet across where water was surging up from below. It was an underwater, freshwater spring.
"People from the National Geographic Society measured the outflow at 3500 liters (925 US gallons) per minute," the guide said. "This spring is connected to the big cenote (sinkhole) we're about to visit inland now where you'll get to swim in clear freshwater with tropical fish all around you. The people from National Geographic dove from here through a cave to a point inland where they encountered a hole through which all this water was pouring, but the hole was only about a foot across and the divers couldn't continue. However, they dove into the inland cenote we're about to visit and followed the streaming water seaward, to the other side of the same foot-wide hole they'd seen coming from here. That way they knew that these two freshwater sources were connected."
It all makes sense. The entire Yucatan Peninsula is basically a big slab of limestone very shallowly tilting toward the sea. No rivers flow on northern, inland Yucatan's surface because the peninsula's limestone bedrock is highly fractured and filled with solution features such as underground caverns. Here water drains into cracks and sinkholes, then runs beneath the land's surface. The Yucatan has a never- ending, peninsula-wide river of freshwater beneath it running from its interior toward its coasts. At the hacienda, wells about 20 feet deep stay filled with freshwater even when we're watering plants all day or filling the pool. If a tree can get its roots down to groundwater, it has it made.
By the way, the peninsula's shallow tilt also explains why hundreds of yards/meters from the beach the water still was only a couple of feet deep.
On the Web you can read a somewhat technical history of the "Yucatan Platform," provided by the Quintana Roo Speleological Survey, with an emphasis on how caves and cenotes were formed, at http://www.caves.org/project/qrss/geo.htm.