Stairs lead to a tunnel and cave entrance along Denecourt Trail number 8 (sentier Denecourt n°8, Le Mont-Aigu) in the Fontainebleau forest of France. The underground passage was created in 1853 by Claude François Denecourt and his team of trail builders. He named it the "Grotte du Serment" (The Oath Cave) because he promised it would be his last ambitious underground trail project. Inscribed at the entrance of the cave are the initials D.F.D. for "dernier folie de Denecourt," (last extravagance of Denecourt). It wasn't long however before Denecourt broke his promise and started work on a new cave, aptly named the "La grotte du Parjure" (The Oath Breaker Cave).