About twenty kilometers from Bonito, in Mato Grosso do Sul, Gruta do Lago Azul harbors an underground lake whose waters take on an intense blue hue when sunlight penetrates through the roof opening. Designated a protected site in 1978, the cave combines geological, paleontological and scenic interest.

Why visit Gruta do Lago Azul

Estimated at over ten million years old, the cavity is one of the largest flooded caves on the planet. Its interior lake, fed by underground waters, displays remarkable transparency and blue coloring on certain days of the year. The walls bear speleothems—limestone concretions with silhouettes evoking winged beings, nicknamed "angels" by locals—supplemented by stalactites and stalagmites.

The cave is also a paleontological deposit: scientific dives have unearthed thousands of Pleistocene animal bones. The site is managed by Brazil's National Institute of Historical and Artistic Heritage (IPHAN) and visitor access is strictly regulated.

History and scientific discoveries

The cave's existence was revealed in 1924. The first known visitor is believed to have been a Terena Indian from the region. The cavity remained little-known for a long time before attracting researchers' attention.

In 1992, a Franco-Brazilian expedition explored the lake and submerged galleries. The dives revealed an exceptional fossil deposit: thousands of bones from giant ground sloths, giant armadillos and saber-toothed tigers, species that coexisted in this territory at the end of the Pleistocene. The South American smilodon, the emblematic predator of the era, measured approximately 2.1 m in length and 1.2 m at the shoulder, weighing 300 kg.

The site has been designated a national heritage site since 1978. Its management by IPHAN ensures the preservation of concretions, fossils and endemic fauna.

What you will see on site

• The circular entrance: the descent to the cave follows a wooded path then approximately 300 steps carved into the rock. The route is physically demanding and not recommended for those with vertigo or claustrophobia.
• The vault and its "angels": limestone speleothems with forms evoking winged silhouettes, complemented by an array of stalactites and stalagmites.
• The blue lake: several tens of meters deep, it takes on a vivid blue color when zenith light passes through the roof opening.
• Endemic fauna: the cave is home to notably Potiicoara brasiliensis, a crustacean first described here and found nowhere else.
• A second sealed cavity: Gruta de Nossa Senhora Aparecida, located on the same site, is no longer accessible to visitors for safety reasons.

Diving in the lake is reserved for authorized researchers. Visitors observe the water body from an arranged platform.

Practical information

Unguided visits are prohibited: an accredited guide must accompany each group. Budget approximately 2 hours on site, plus travel time from Bonito. The trail includes slippery sections; wear closed-toe shoes with good grip, bring water and light clothing.

Entrance fees vary by season and do not include transportation. The site enforces a daily visitor quota. It is best to book in advance through a local agency or tour operator, particularly during peak season (July and December to February).

When to visit

The cave is accessible year-round, but the "blue lake" effect depends on the sun's angle. The optimal period runs from late December to early February: the sun at zenith penetrates widely through the opening and illuminates the lake between roughly 9 a.m. and noon. For the rest of the year, the blue coloring remains visible but less spectacular.

If possible, avoid days following heavy rains: water may become cloudy and the access track becomes difficult.

How to get there

The cave is located about 20 km northwest of Bonito. The road is partly unpaved: passable by standard vehicle in dry season, it often requires a 4x4 after rains. Most visitors book an excursion that includes transfer and guide from central Bonito, the simplest solution given the mandatory guide requirement.

Nearby attractions

Bonito concentrates other remarkable natural sites, easily combined over two to four days:

• Rio da Prata and Rio Sucuri for floating in crystal-clear waters (snorkeling).
• Abismo Anhumas, a 72 m sinkhole accessible by rappelling, with diving or floating in an underground lake.
• The Buraco das Araras, an immense sinkhole populated by red macaws, about an hour's drive away.

Frequently asked questions

Planning your trip

Gruta do Lago Azul fits naturally into a circuit through Mato Grosso do Sul, often combined with the Pantanal. To build a custom itinerary, consult our trips to Brésil.