Attack of the bark beetles

I walked up to the lake behind the house, 2 km away, which was still frozen.
Just as I got to the lake I found this tree which had been long dead but this winter the bark had fallen from the trunk, revealing this pattern of tunnels, where bark beetles have been at work.
Bark beetles usually infect dead and dying trees but sometimes they attack healthy trees too. I've no idea which this was when the beetles got to work!