Earlier, my little brother and his friend started playing Total Miner for the first time. Before so, their friend told them that if they stay on the surface for a period of time, a powerful monster that digs through dirt would come and kill them. They spend the entire day gathering materials for a bunker to protect them. I told them it wasn't true, but it gave me an interesting idea.
How about in random rock layers, starting at 500 ft, one of these said monsters would spawn and dig through that layer. Only one of them would exist per layer and can only dig through the material the layer they spawned in has. It would wonder around aimlessly until it either encounters a player, or if a player spends an allotted amount of time in that layer, then the monster would go straight for them unless they retreat to another layer. They would leave a tunnel where ever they went that would disappear after a period of time. Once they were dead, they wouldn't come back unless physically spawned.
I think that it would give Dig Deep some intensity and fear. What do you guys think?