After seeing some effort for getting Minecrafting skill from 25% to 101%, I created my first abandoned merchant mines (to Lucentium continent). My first mine gave me only about 40k exp, but from the second mine I got about 300k500k.

You feel like you've gained some experience for your efforts to the explorers of this realm.
Achievement unlocked (A player kills monsters in your abandoned mine.)

 *** You get exp! And you get exp! EVERYBODY GETS EXP! (100) *** 

In addition, after maxing out Chest creation, I have created six 12-slot vaults. A few more and I should again have enough chest space for a while.

Update 2026-03-20: I actually did get almost 500k exp from the mine in a boot. Someone did more killings while I had already went offline.