Aern Hollow is a natural depression in a large Northumbrian forest, and the site of a vast network of underground tunnels. These tunnels were originally a medieval tin mine but were converted into a secret wolfblood den by the Smith family many hundreds of years ago.
The tunnels lead all the way to the Den in the basement of the Smith family house, though at some point in the past this passage was bricked up - perhaps after the location of the tunnels was discovered by Abraham Runcorn Hunter around 1800 AD, and the Smith family feared detection. Daniel Smith had no knowledge of the tunnels under the house until their re-discovery, though he knew that there were medieval tin mines in the area.
The name comes from the Old English word ærn, meaning 'house, habitation' (a reference to the lair) and hollow, meaning a depression in the landscape.
In around 1800, Abraham Runcorn Hunter pursued a massive wolf through the forest - William Smith in wolf form - and suspected he had disappeared into a den in the area. Abraham never found the tunnels himself, but documented their suspected location in his journal. He was able to shoot and kill William with a silver bullet at a later time on his own land (Grave Consequences).
Liam, Jimi and Sam explore the Hollow in Going Underground, and find the tunnels lead to a basement, where the Smith family are in wolf form for the full moon. Liam returns in The Discovery and sees it is the basement of the Smith family house - and believes they are werewolves, just like his ancestor used to hunt.