The Acqua Vergine runs for a total of 20 kilometers and ends up in the Trevi Fountain, photographed every day by crowds of tourists.
"Underground Rome is a final frontier," said Riccardo Paolucci, another explorer, as he examined a viaduct in a valley near Vicovaro that carried the water further towards the city.
"Water was a fundamental service for hygiene. In a city like Rome, which had a million inhabitants, there were very few epidemics," he said.
"There was a concept of service for the people, for the city. It is a key concept that is maybe lacking not just in modern Rome but globally."
Diaz, Paolucci and the others from Sotterranei di Roma work together with Rome's archaeological authority, helping them understand what can be seen above the ground from what is underneath and inaccessible without specialist equipment.
"We are who we are because of what we have inside and Rome is what it is because of what is underneath it," said Paolucci, a specialist potholer who is also called to emergency incidents or whenever a sinkhole opens up in the city.
The group also organizes guided tours and courses, including one on the aqueducts starting next month, and are earning an international reputation. They were commissioned to map the underground remains of ancient Ephesus in Turkey.
Their study of the aqueducts is based on the map made by Ashby, director of the archaeological British School at Rome between 1906 and 1925.
Ashby's signature can be seen scrawled on the wall of a section of the Acqua Marcia aqueduct, which also transits through Vicovaro, alongside graffiti and poems dating back to the 17th century from visitors who stumbled on the ancient waterway.
"Ashby's maps were ahead of their time," Diaz said.
"He want to the villages, to the local trattorias, he spoke to farmers, to hunters. He found what he found thanks to local knowledge," he said.
"It is a technique that we still use today."