The cave contains approximately 120 engravings, 92 of which depict animals. The artists worked during the Upper Magdalenian period, at the end of the Upper Paleolithic, etching their subjects into the rock walls with a realism characteristic of the Franco-Cantabrian School. Bison dominate the collection with 53 representations, an emphasis that suggests the animal's central role in the lives and imaginations of the people who lived along the Cantabrian coast during the last Ice Age. The presence of saiga antelope is particularly striking -- these are animals of the cold grasslands, evidence that the Basque Country's climate was once dramatically different from the mild, rainy landscape of today.

In 2013, researchers dated the paintings in the upper gallery, Altxerri B, to approximately 39,000 years ago, placing them in the Aurignacian period. This made them the oldest known stone paintings in Europe, challenging the primacy of France's Chauvet Cave. The discovery pushed back the timeline of artistic expression on the continent and demonstrated that the earliest European artists were working across a wider geographic range than previously understood. The paintings in the main gallery date to the later Magdalenian period, meaning the cave was used for artistic purposes across tens of thousands of years.

Altxerri is closed to the public. The decision to seal the cave reflects hard lessons learned from other prehistoric sites, where tourism degraded fragile cave environments. In 2008, the cave was declared a UNESCO World Heritage Site as part of a group of 17 caves in northern Spain collectively designated as the Cave of Altamira and Paleolithic Cave Art of Northern Spain. The designation recognizes these caves as an extraordinary record of early human creativity. Inside Altxerri, the engravings remain as their makers left them -- precise, expressive, undisturbed -- a bestiary from a world of ice and megafauna that existed long before the Basque people, or any modern people, walked this coast.