This should hopefully teach you how to do the Easteregg on the Belgium map, this was (i think) discovered by me. And i am sharing how to do this.
Other Unturned Guides:
The location of Simon is west of Brussels, near a thin curvy road. Inside a shed.
One is in Brussels, at the castle under a yellow tent.
The next is at Konnoke Beach, washed up on the shore.
The last is north of Florrens Military base, at a tiny island lodged between two rocks.
Cave, and Magma Mere
Simply, return to Simon. Then accept his next quest.
The cave you are looking for is at Singal de Botrange. And is on the cliffside.
Once inside, kill the boss and claim Magma mere, then leave the way you entered.