SEE | Mystery stone coffin believed to be from the 14th century found in Notre-Dame after the fire

A mysterious leaden sarcophagus discovered in the bowels of Paris' Notre-Dame cathedral after it was devastated by a fire will soon be opened and its secrets revealed, French archaeologists said Thursday.

The announcement came just a day before the third anniversary of the inferno that engulfed the 12^th century Gothic landmark, which shocked the world and led to a massive reconstruction project.

During preparatory work to rebuild the church's ancient spire last month, workers found the well-preserved sarcophagus buried 20 metres underground, lying among the brick pipes of a 19^th century heating system.

A 14th century lead sarcophagus discovered in the floor of Notre Dame Cathedral.

JULIEN DE ROSA / AFP

But it is believed to be much older - possibly from the 14^th century.

Scientists have already peeked into the sarcophagus using an endoscopic camera, revealing the upper part of a skeleton, a pillow of leaves, fabric and as-yet unidentified objects.

The sarcophagus was extracted from the cathedral on Tuesday, France's INRAP national archaeological research institute said during a press conference.

It is currently being held in a secure location and will be sent "very soon" to the Institute of Forensic Medicine in the southwestern city of Toulouse.

Forensic experts and scientists will then open the sarcophagus and study its contents, to identify the skeleton's gender and former state of health, lead archaeologist Christophe Besnier said, adding that carbon dating technology could be used.