Although many know the magnificent Cologne Cathedral as the city's main tourist attraction, few are aware that it's also a major attraction for archaeologists.

"We've uncovered a huge area here and worked our way back to the ancient Roman period," said archaeologist Ruth Stinnesbeck, with an area of around 4,000 square meters (43,055 square feet) under the famous church being explored.

The well-preserved finds discovered underground at depths of between 3.5 to 16 meters (37-172 feet) under the cathedral have thrilled Stinnesbeck and her colleagues. "It's unlike everywhere else in the city, where old buildings are constantly being replaced by new ones and little remains underground," she said.

The extensive excavations under the Gothic cathedral began in 1946. Archaeologists had hoped to start exploring much earlier, but this would have required tearing up the church's floor, which was deemed to be too disruptive to everyday church life and therefore not permitted.

However, a year after World War II came to an end, archaeologists were encouraged to do their work for another reason. "They wanted to check whether the cathedral's foundations had survived the bombing. The cathedral was badly damaged and normal operations were suspended," explained Stinnesbeck.

A popular spot to build churches

Originally, the current team of archaeologists wanted to learn more about an older cathedral building dating back to the 8th and 9th centuries that was no longer standing. Historical records told them it had once stood in that very spot — and had an important significance.

Charlemagne, the Frankish king who later became Holy Roman Emperor, had appointed his close friend and adviser Hildebold as the first archbishop of Cologne around the year 795. To celebrate the appointment, he built a Romanesque-style cathedral on the site where a baptistery had already existed for two centuries.

Although it was nowhere near the size of the Cologne Cathedral, which is one of the largest churches in the world, the former building was by no means small, said Stinnesbeck. "It was almost 100 meters (328 feet) long, which was very significant 1,200 years ago — that's two-thirds the size of today's cathedral," she said.

Ancient Roman luxury

However, deep underground, archaeologists discovered not only Hildebold's building, but also much older historical finds from the time of Roman rule. The Roman period in Cologne lasted from the 1st century BC until the mid-5th century AD — around 500 years.

Initially, present-day Cologne was merely a Roman colony for wealthy families in the Rhineland called Colonia Claudia Ara Agrippinensium. Remains of their luxurious villas are currently being unearthed beneath the cathedral, including murals and even traces of an underfloor heating system. 

"The Romans had a good time here in the cold northern provinces of the empire," said Stinnesbeck. Although the ruins below ground don't exactly look impressive to the untrained eye, with a little imagination they bring the past alive, she added. 

"You have to imagine two wicker chairs, a small marble table and a bowl of olives," she said, peering into the ruins. She explained that one particular villa was located near the Rhine River, as is the cathedral, but was elevated and therefore flood-proof — certainly a great privilege at the time. Wine, a popular ancient Roman beverage, was also grown here.

'Ancient recycling'

The city was taken by the Ripuarian Franks in 462 AD, and the name Colonia Claudia Ara Agrippinensium was lost. "But the hustle and bustle of life in this area continued more or less seamlessly under the Franks," said Stinnesbeck, adding that historical evidence of this period was also found during the excavations.

Relics dating back to Roman times have helped piece together a portrait of life in the past, she said. 

"When the cathedral was built, pieces of remaining buildings here were used in its construction — ancient recycling, so to speak. We may have reinvented the word, but the reuse of building materials has existed throughout history," said Stinnesbeck.

Coarse stones from Roman times were used to support the foundations of the 9th century Hildebold Cathedral. "Finer objects such as porphyry, a decorative stone in bright red or green, on the other hand, were often reused in prestigious places of the church" she said.

How stable is the cathedral?

The extensive excavations under the cathedral have revealed a great deal of interesting information about antiquity, but many visitors wonder if such large-scale work might affect the building's stability.

"The opposite is the case," assured Stinnesbeck, adding that structures have been put in place to ensure that the cathedral is properly supported.

In 2008, two tunnels were drilled through the cathedral's masonry, which is more than 11 meters (36 ft) thick, to provide access to the tower and the underground area from outside the cathedral.

As a result, visitors can now climb the south tower of the cathedral and take a peek at the ongoing excavation. They might even have the chance to meet Stinnesbeck, who occasionally gives guided tours.

"Even as an archaeologist, I have to acknowledge that visiting the inside of the cathedral is, of course, much more impressive — but what's happening underneath is much more exciting."

This article was originally written in German.