The workers had just begun to improve the water supply in the arid southern province of Seville.
They hadn't dug deep yet when an archaeological sensation stopped them: in the heart of Andalusia they came across the largest Phoenician-Punic necropolis in inland Spain to date.
The stone tombs date from the fifth and fourth centuries BC.
The small town of Osuna now stands on the site of the former Roman town of Urso.
According to an initial assessment by the regional Ministry of Culture and Historical Heritage, these are "remains of undeniable value" that are "unprecedented" in the region for their age and state of conservation and can at best be compared to previous finds in the coastal area of Cadiz.
In 1100 BC, the Phoenicians, who came from the eastern shore of the Mediterranean, founded their trading post at Gadir, which is considered to be the oldest city in Spain.
According to initial findings, the burial complex at Osuna shows parallels to necropolises in Corsica and Tunisia.
It is a "very remarkable find, since a necropolis of this type is rarely found so far inland and is otherwise typical of coastal areas," says excavation director Mario Delgado.
Eight subterranean vaults carved into the calcarenite rock have been uncovered so far.
The luxurious tombs give rise to the assumption that wealthy people were buried there.
They were then built over in Roman times.
The discovery could mark a turning point in Osuna's history, providing new evidence that it dates back to the eighth century BC, says the city's mayor, Rosario Andújar.
Archaeologists had previously made finds from the Iberian period in the area.
Later historians such as Pliny and Ptolemy reported Urso unsuccessfully fighting on the side of Pompey against Julius Caesar.
Today, a good 17,000 people live in the small town in southern Spain, almost 90 kilometers west of Seville.
Game of Thrones fans know the location: parts of season five were filmed on the streets and in the bullring.