How is the data in a MongoDB database stored on disk?

I know that MongoDB accepts and retrieves records as JSON/BSON objects, but how does it actually store these files on disk? Are they stored as a collection of individual *.json files or as one large file? I have a hunch as to the latter, since the MongoDB docs state that it works best on systems with ext4/xfs, which are better at handling large files. Can anyone confirm?