The caves are carved into the Gaztelu hill, a limestone prominence in the foothills where the Pyrenees descend toward the Bay of Biscay. Two main chambers - Isturitz above and Oxocelhaya below - were formed by underground rivers cutting through the rock over geological time. The upper cave, Isturitz, held the primary living spaces. Its large chambers offered shelter from Atlantic storms while remaining close to the hunting grounds of the Pyrenean foothills. Neanderthal communities were the first to recognize these advantages, leaving stone tools and a mandible that confirmed their presence. When Homo sapiens arrived, they inherited a shelter that their predecessors had used for millennia and made it their own, layering new tool traditions and artistic expressions onto the archaeological record like pages in a book written by different hands.

Cave paintings adorn the walls, placing Isturitz among the easternmost decorated caves in the Franco-Cantabrian tradition - the great chain of painted caves that extends along the Pyrenees and Cantabrian Mountains from Altamira in Spain to sites scattered across southwestern France. The paintings share the naturalistic style characteristic of this tradition: animals rendered with close observation of musculature and movement. Engravings on stone found in the cave include a wounded ox with a weapon visible in its side, a scene that captures both the hunting life of its creators and their artistic skill in rendering the aftermath of a hunt. A small statuette, discovered in 1896 but subsequently lost, appeared to represent a cave lion cub, one of the predators that shared this landscape with its human inhabitants during the Late Pleistocene.

The caves yielded an extraordinary range of worked bone and antler objects. Gravettian projectile points demonstrate sophisticated weapon-making skills, while baguettes demi-rondes - half-round spear tips engraved with spiral patterns - show that functional objects were also canvases for decorative expression. The bone flutes are the most evocative finds. Multiple flutes were recovered, their finger holes carefully spaced along hollow bird bones, instruments capable of producing distinct musical notes. They belong to a tradition of Paleolithic music-making found at sites across Europe, but the concentration at Isturitz is unusual, suggesting this cave may have held particular significance as a gathering place where music played a role in social or ritual life.

The French government classified Isturitz and Oxocelhaya as a Monument historique in 1953, recognizing seventy millennia of human presence in a single site. The lower cave, Oxocelhaya, features spectacular natural formations - stalactites and stalagmites that the underground river sculpted over millions of years - alongside the archaeological deposits above. Together the two chambers preserve a nearly complete sequence of Paleolithic cultural traditions from the Mousterian tools of the Neanderthals through every major phase of Upper Paleolithic development. Some evidence suggests the caves continued to be used for burials even after permanent occupation ceased, with traces extending into the Roman period. Human bone fragments found at the site include pieces that show signs of deliberate modification, raising questions about funerary practices that researchers continue to investigate. The caves remain open to visitors, their chambers holding the accumulated traces of lives lived across a span of time that dwarfs recorded history.