Gruta da Lagoa Azul State Park, Natural limestone cave system in Nobres, Brazil

Gruta da Lagoa Azul State Park covers 12,513 hectares of deciduous forest in Nobres, Mato Grosso. Its main cave holds a blue pool fed by the Saloba River; the entrance is partly flooded. The chamber contains columns over 5 metres tall and 1 metre in diameter, plus stalactites, stalagmites and curtains.

The cave system became a protected area in 1999 when the government established the state park through a formal decree. This action expanded protection from an earlier reserve to cover a broader conservation zone.

The cave's name comes from the blue pool fed by groundwater of the Saloba River, which partly fills the main entrance. Residents of Nobres knew the cave was badly vandalised, with broken speleothems and carvings and writing on the walls, and backed its closure until a protection plan existed. A management plan later identified 14 types of bacteria harmful to humans in the cave's soil, water and air.

Public visits were suspended by an ordinance of 30 May 1999 until a management plan was ready, and the park remains closed to visitors. The cave's ecosystem depends on constant temperature and humidity, so even visitors' breath and lamp heat can encourage fungi and lichens on the speleothems.

The cave's management plan identified 14 types of bacteria harmful to humans in its soil, water and air. Visitors once left carvings and writing on the walls, and broken speleothems remain as evidence of that damage.