see http://docs.mongodb.org/manual/reference/method/cursor.sort/#return-natural-order

> The $natural parameter returns items according to their order on disk. Consider the following query:

> db.collection.find().sort( { $natural: -1 } )

> This will return documents in the reverse of the order on disk. Typically, the order of documents on disks reflects insertion order, except when documents move internal because of document growth due to update operations.