Some 15 metres below ground and 200 metres long, the cavers came across the grotto a year ago, but have kept it secret until now.
Other adventurers say they had suspected there was a cave in the area for a few months.
“When we came in for the first time, it was really mythical… we wanted to laugh, to shout, to cry all at the same time. It’s magical,” said one potholer to TV station France 3.
“There is no artist who could even imagine this,” said another caver.
Earlier this month, the caving group notified the Mayor of Arbois, a nearby town that owns the land where the grotto was found.
For now, the cave will be closed to the public until official testing to determine exactly how old it is has been completed.