The implementation principle of 2dspere index in MongoDB?

  mongodb, question

The Mongo document introduces that its 2d index is implemented by GeoHash, but it does not mention the implementation principle of 2dsphere, and no introduction can be found on the network.
Does anyone know the implementation of 2dsphere?

B+ tree index.

I am also looking for the answer to this question, so Google has been searching for a long time and finally found a paper in July 2016:
Providing R-Tree Support for MongoDB – ISPRS Archives
This paper describes how to use R-tree to realize mongodb’s spatial index. When it comes to 2dshpere and the basically abandoned 2d index, both are spatial indexes realized by B+ tree.
I hope someone can check the source code (funny).