Astronomers have discovered what they believe could be "the first astronomical observing tool", used by humans 6,000 years ago to observe the stars.
The tomb's design suggests it may have "enhanced what early human cultures could see in the night sky" and a single aperture, such as a window or doorway, can affect the observation of stars, the team found.
The study was focused on passage graves at Carregal do Sal in Portugal, a "type of megalithic tomb composed of a chamber of large interlocking stones and a long narrow entrance".
The light from stars would then have been observed by a group of humans who had been "initiated" into what they believed was a sacred practice.
"The orientations of the tombs may be in alignment with Aldebaran, the brightest star in the constellation of Taurus," said Fabio Silva, who worked on the project.
"To accurately time the first appearance of this star in the season, it is vital to be able to detect stars during twilight."