La Media Luna is a thriving wetland of considerable ecological diversity. The lake's warm water (constant temperature of 28 - 32 degrees C) and visibility of 20 - 30 meters 365 days a year make it an ideal site for scuba diving, especially dive training. La Media Luna is located at the far western edge of what is known as the Huasteca Region, a region generally known for outstanding rafting, rock climbing, kayaking, and exploring sites off the beaten path to ancient ruins. La Media Luna is nestled in the high semi-arid plateau of the Sierra Madre Oriental Mountains in the State of San Luis Potosi (SLP). The entire basin is often referred to as the Rio Verde Valley (Green River Valley). In prehistoric times the valley was an extensive lake covering the plateau. Fossil records found in-and-around the lake reveals that mastodons and other prehistoric creatures were frequent visitors to the clear waters of La Media Luna. Based on artifacts discovered in the lake, ancient indigenous ceremonies were performed as early as 800 BC. Engineers have attempted to mechanically tap the clean water but each attempt has failed. Today, concrete pedestals once used to support huge pipes that carried pumped water from the lake have been converted to picnic table legs (supports). The overflow is now channeled by gravitation to more than 5,000 hectares of farmland for irrigation hence changing a mountain desert-land into a citrus fruit and vegetable bonanza.
The lake's water is extraordinarily clear. The mineral content is a little higher than most freshwater and this contributes to a slight increase of positive buoyancy for divers. The lake is fed by six ancient thermal karst springs at depths of 10 to 38 meters with a flow rate of 4.35m3/sec. There are at least ten species of fish, most are native and several are endemic to the area.
Most interesting is Cichlasoma bartoni, a cichlid, called the "skunk fish" by locals because during it's mating season it becomes a black fish with a white stripe along its dorsal area. The bottom of the lake is covered by a thick layer of detritus with several small patches of sand. The detritus requires divers to dive with care, trying not to disturb the bottom. If it is disturbed, the visibility in and around that spring will become cloudy and will require 20 - 30 minutes to clear up. If one spring does become disturbed the visibility of areas around the other five springs remains clear.
The aquatic vegetation in the shallow waters consists mainly of beds of water lilies. It's a fascinating experience to watch the gases bubble to the surface from the underwater leaves of the lilies as fish and invertebrates hide under and around the leaves.
From the parking lot, there is a well maintained boardwalk that leads to the lake. At the end of the boardwalk is a large dock that is used by divers as a starting point. One giant stride and your in the warm water with about 10 meters of water below you. As one dives along the west side of the lake at about 10 - 15 meters an ancient petrified underwater forest is exposed. Huge trees, perhaps cypress, fell into the lake long before man arrived and became petrified in the mineralized water. If a diver wants to do some deep diving, the deepest spring is 38 meters (about 118 feet) down and is located about 50 meters from the dock (lamp required). The water rushes out of the spring at around 33 degrees C or warmer (that's somewhere in the area of 92 degrees F). Another exciting and unique dive experience is a small cave-like opening along the southwest banks of the lake. It;s not really a cave as we usually think of caves. The ceiling is made up of surface plant roots. Openings through the ceiling allow light to enter so you are never in the dark. The diver is only 3 to 5 meters deep while traveling through the cave-like tunnel. At approximately 50 to 70 meters there is a huge cavern which is usually attended by turtles.
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