A research team was combing the coastline of southwest Portugal in 2019 in search of signs of how its ecosystem had changed over time. They stumbled upon an astonishing and unexpected scene: bees that had been mummified in subterranean sarcophagi for nearly 3,000 years.

An unlikely series of events had conspired to preserve this helpless horde of pollinators over millenniums. Whatever had happened, it was “an unlucky night for hundreds of adult bees that were ready to leave their cocoons,” said Carlos Neto de Carvalho, a paleontologist and scientific coordinator at the Naturtejo UNESCO Global Geopark in Portugal.

These were not typical European honeybees in a hive, but members of a group called Eucera. The bees spend most of their yearlong lives gestating underground and eating pollen left by their mothers, emerging for only a few weeks when their favorite flowering plant blooms.

Considering the clues, Mr. Neto de Carvalho and his colleagues deduced that the bees had most likely met an abrupt demise.