The Deacon’s Bench links to some pictures of France’s oldest oak and the two 1696 treehouse chapels built in it.
The French Wikipedia article has a lot more info. (And pictures at the bottom.)
Back in 1696, the abbe of Detroit, who was also the parish priest, was interested in seeing how stable the hollow oak was and how big. So he paid the local kids to see how many of them could stand inside the tree. Once 40 kids fit inside, he figured it was big enough to be used as a tree chapel. So he built a chapel to Our Lady of the Country on the “second floor”, and a “third floor” hermitage/cell and “Calvary Chapel” for his buddy, Fr. Du Cerceau, who wanted to be a hermit. So his friend lived there until his death. His bedstead is still there, but nobody else has taken his place as village hermit. He wrote a poem about the oak and about a second chapel the Abbe built in a famous hollow thorn tree that was destroyed in the Revolution.
At that time, the church was partly surrounded by oaks, not by buildings. But the French navy was as hungry for oaks as any other, so the rest of the oak grove is gone.
The chapel and the tree were subjects of a lot of legends in the surrounding area. So come the Revolution, the tree was to be burned or chopped down. However, the village schoolmaster quickly made a sign declaring the chapel a “Temple of Reason” (as many churches in Paris had been named officially, to take them away from the Church); and apparently the government forces were appeased or frightened off by this.
The village kept the chapel and the oak, and eventually it drew the attention of Empress Eugenie, who gave them a gilded oak statue of the Virgin for the chapel. (Now in a safer place.) The government then encouraged the village and its priest with funds to help fix the “historical monument”. The chapel was carefully restored, given a new altar in a style the abbe would have liked, and reconsecrated by the bishop in 1854.
In 1912, the oak was struck by lightning, and half of the tree fell off. That’s why the back part looks kind of bad. The village installed all sorts of contrivances to help support the tree, and it is constantly checked. The chapel contains a wood altar, a couple of candlesticks, and pasted up pictures of Mary, St. Joseph, and St. Jean-Francois Regis (a famous evangelizing French Jesuit who died in 1640).
The chapel is used for two official Masses a year, and there is a pilgrimage to the oak on the Feast of the Assumption of the Blessed Virgin Mary, every August 15th.
The oak also starred in a fictional 1981 comedy movie about efforts to save the oak. It is over 1100 years old.
The Wikipedia article links to two other French oak chapels: St. Sulpice-le-Verdon (St. Sulpice in the Green Field), built in a living oak, and Chapelle St. Joseph du Chene (St. Joseph of the Oak) in Villedieu-La-Blouere, built around an oak that is now dead. The trunk is still standing strong above the statue of St. Joseph.
UPDATE: For everyone who thinks Catholics are godless pagans, relevant Biblical quotes!
“But a shoot will sprout from the stump of Jesse.” (Is. 11:1) [Hence the association with Mary, as part of the “Jesse Tree” of ancestors leading to Jesus’ birth.)
“He set it under the oak that was in the sanctuary of the Lord.” (Josh. 24:26) [Comparing Mary’s womb to God’s Temple.]
“All the trees of the country shall clap their hands.” (Is. 55:12) [Hence “Our Lady of the Country.”]
“Then shall the trees of the wood give praise before the Lord.” (1 Chron. 16:33)