Excellent video

This ones a DOOZY!

This is what Greg wrote about this particular plug and the reason for why its tanned all the way down to the eventual boundary layer:

BP 98 fuel.. (That's a premium pump fuel)
Now car has no pinging ...
Clean all the way to the bottom.. that means that as the afr gets closer to the head it’s still well mixed... as you get closer to the boundary layer near the chamber wall it gets colder usually and that changes the burn and the gas type that gets produced from the burn at that point .. you can see this on plugs..