This reminds me of an episode of Map Men where they offhandedly point out that by plotting British place names of Viking origins, it is possible to see the true extent of their invasion: https://youtu.be/uYNzqgU7na4?si=l-b6-p4fSeviWmMs&t=310. In fact, the map they show in that episode (https://www.reddit.com/r/MapPorn/comments/n2s2qu/map_of_brit...) looks strikingly similar to this map I've found on Wikipedia of Celtic, Danish and English territory in 878: https://en.wikipedia.org/wiki/File:England_878.svg