Molerats

These pests have a nasty habit of burrowing into your Vault's dirt and chewing on any room's power cables. During a molerat infestation, your Vault's power supply will steadily drain. Like Radroaches, they'll spread quickly to other rooms. They're sturdier and more damaging than radroaches. A molerat infestation can literally wipe out an entire vault if its dwellers have no / few Weapons and stimpacks. They become significantly less threatening as your dwellers level up and gain access to more powerful weapons and even more supply of stimpacks.