Archaeologists on the Iberian Peninsula have discovered a remarkable 65,000-year-old tar-making factory built by Neanderthals.

A new study published in the journal Quaternary Science Reviews revealed that this feat was accomplished 20,000 years before modern humans (Homo sapiens) arrived in the region.

Neanderthals used sticky tar to create glue to make weapons and tools. The entire process took place in a carefully designed hearth that allowed them to regulate the fire and control the temperature to create the sticky substance.

Although archaeologists have long known that Neanderthals produced glues, including tar, resin, and sticky substances from ocher, this newly discovered hearth, likely built into the floor of a cave in what is now Gibraltar, reveals that Neanderthals were skilled engineers who had refined the process of making glue.

The researchers wrote in the study, “The structure has revealed a previously unknown way by which Neanderthals managed and used fire.”

The Neanderthal hearth consists of a circular pit about 09 inches wide and 3.5 inches deep (22 x 9 centimeters) with sharply defined vertical walls and two small ditches, about an inch long, running north and south.

The initially seemingly simple hearth is actually a remarkable feat of precision engineering that leaves scientists in awe.