Typically, a database storage layer is built on B-trees. The same applies to WiredTiger - the default engine for MongoDB.
However, Percona Server for MongoDB has a specific engine built on an LSM tree - it's MongoRocks. Based on the RocksDB library from Facebook, MongoRocks became a different but interesting storage layer solution.
This talk covers its implementation details.
Denis is working at Percona on the Percona Server for MongoDB side.