It’s an incredible finding — a series of 6,000-year-old tombs in Portugal hold a fascinating secret about ancient man, indicating that humans were building stargazing tools long, long ago.

A study presented at the National Astronomy Meeting in England this past week showed evidence of simple apertures like tunnels and doorways that may have been used to observe the stars, according to NAM statement.

If the findings are correct, these 6,000-year-old tombs would be the oldest astronomical observatories ever discovered.

The research team believes the narrow entrances of these graves increase the visibility of specific stars, allowing stargazers inside to be fully cloaked in darkness to make the stars in the field of vision more visible.

And it wasn’t for curiosity’s sake, scientists believe. The early stargazers would have been able to witness the first rising of the star Alderbaran at the end of April or beginning of May, which would tell them that it’s time to move to higher grounds, as these civilizations lived near the river in the winter and in the mountains in the summer.

Scientists will continue to examine these passage graves to confirm that this is what they were used for.

“The first sighting in the year of a star after its long absence from the night sky might have been used as a seasonal marker, and could indicate for example the start of a migration to summer grazing grounds,” the statement reads. “The timing of this could have been seen as secret knowledge or foresight, only obtained after a night spent in contact with the ancestors in the depths of a passage grave, since the star may not have been observable from outside. However, the team suggest it could actually have been the result of the ability of the human eye to spot stars in such twilight conditions, given the small entrance passages of the tombs.”