An ancient mosque, the oldest in Europe, is believed to be buried underground in Reccopolis, on the outskirts of Madrid in Spain. Using a geomagnetic instrument, archaeologists have detected walls and other structures of the monument.

Michael McCormick, a medieval historian and archaeologist at Harvard University told Live Science that they were able to survey every space. They found buildings, passages and streets. The ancient city, also known as ‘Muslim Atlantis’ and about 1,400-years-old, has hidden secrets. According to GROMA, the city of Reccopolis was projected as an ex-novo foundation and an explicit political power statement. It says that for two and a half centuries, from the Visigothic period until the early Islamic period in the mid-9th century, the city was the center of production and consumption.

Historians say the Visigoth era was overshadowed by Roman rule. Medieval history describes early Visigoths as a Germanic group that alternated between opposing and serving the Roman Empire. In 410 c.e., the Visigothic King Alaric sacked Rome, settled in southern France and spread into Iberian Penninsula. The Visigoths were later defeated by the Catholic Franks. In the year 711, an Arab invasion further weakened the Visigoth kingdom.

Years of archaeological works spanning from the 1940s to date, have revealed a hierarchical urban plan. However, excavations have only uncovered 8% of the area.

In 2014, when McCormick visited the site, he found ruins of a palace, a chapel and some shops. Wanting to know about the rest of the city, he teamed up with fellow researchers and colleagues and set out to do the first geomagnetic survey of the site in 2015. Through geomagnetic survey, researchers are able to see structures buried deep under the ground by mapping magnetic anomalies beneath the Earth’s surface. According to Live Science, McCormick and his team’s findings revealed that empty spaces inside the city walls of Reccopolis were full of hidden streets and buildings.

McCormick said the Visigothic era witnessed mass migration, food shortages, imperial collapse and famine. McCormick drew attention to the possibility of an ancient mosque since the Visigothic was invaded by the Arabs.