Technical
How does this work? Valheim terrain is a heightmap. Only until you dig. The first dig in a 64×64m zone turns it into a 3D grid of rock and air and meshes that instead. The README has the longer version.
Doesn't that make saves huge? No. Only the list of digs is saved, a few dozen bytes each. The grid is rebuilt from them on load.
What does it cost? Untouched zones cost nothing, and running around converts nothing. A dug zone holds a few MB in memory. The work is spread over frames, and it starts as you swing.
Will it lag my server?
It shouldn't. Only dug zones near players do any work, on a small budget per frame. Turn on Debug logging in BepInEx
if you want real numbers.
Why did digging near a zone border convert the zone next door? Tunnels need both sides converted to cross a border without a seam.
Two of us dug the same spot and saw different results. Known quirk. It sorts itself out when the area reloads.
Can I undo digging?
hearthbelow restore wipes every dig in the zone you're in and buries anything built in its caves.
hearthbelow restore [radius] only fills the digs near you. Admin only on a server.