The mining network is in León province, near the spectacular Las Médulas region, which is considered to be the largest opencast goldmine of the Roman Empire.
It contains evidence that the Romans diverted rivers to supply the mines with water in the first century BC. The network of mines, had, however, remained hidden and overgrown for centuries.
Researchers from the University of Salamanca made the discovery using a remote sensing technology known as LiDAR during an aerial survey.
According to news site Science Daily, LiDAR works by illuminating targets with a laser beam to measure distance.
University of Salamanca geologist Javier Fernández Lozano is co-author of a paper on the discovery, published in Journal of Archaeological Science. "Unlike traditional aerial photography, this airborne laser detection system allows the visualization of archaeological remains under vegetation cover or intensely ploughed areas," he said.
LiDAR was developed by NASA in the late 1960s to track retreating Arctic ice and ocean composition. It can be used in planes or drones.
Fernández Lozano noted in the paper that the Roman mining study in the Eria valley is the first piece of 'geo-archaeology' performed with LiDAR in Spain.
"Our intention is to continue working with this technique to learn more about mineral mining in the Roman Empire and clear up any mysteries such as why Rome abandoned such a precious resource as gold from one day to the next," he wrote.