http://coveryourtracks.iykpqm7jiradoeezzkhj7c4b33g4hbgfwelht2evxxeicbpjy44c7ead.onion/learn
This is equivalent to tracking a bird by its song or feather markings, or a car by its license plate, make, model, and color. In other words, metrics that are harder to change and impossible to delete.