Ojo Guareña

Ojo Guareña is an underground cave network in the municipality of Merindad de Sotoscueva in northern Spain, running through 110 kilometers (68 miles) across multiple levels of limestone and dolomite rock. The galleries connect numerous chambers and passages, some following rivers that still flow through the stone today.

The caves were systematically explored in 1956 by the Edelweiss Group, with archaeologists uncovering tools and remains showing human use since prehistoric times. Different peoples across the Iberian Peninsula sought shelter, water, and refuge here over thousands of years.

The San Bernabé cave holds murals painted in the early 18th and late 19th centuries that retell the life of a Christian martyr, created by hermits who once lived here. Prehistoric rock art shows that people used this place as a shelter or ceremonial site for thousands of years.

The temperature stays between 7 and 9 degrees Celsius (44 to 48 degrees Fahrenheit) all year, so warm clothing and sturdy footwear are necessary. Reservations are required as visitor numbers are limited and only guided tours through certain areas are available.

Researchers have identified 190 animal species in the caves, including 16 invertebrates found nowhere else on Earth. Several organisms were only discovered and described during exploration of this underground world.