I was playing StarCraft II, and come across a new unit called the Widow Mine in multiplayer.
I don't know what it does, all I know is that it can burrow like a zerg.
The Widow Mine is a unit that is built in the Factory. They burrow and act as a proximity mine.
When a unit gets too close, the mine shoots a missile at that unit, dealing huge splash damage (even to your own units). The mine is invisible when burrowed but can be killed easily with detection and the right units.
After the mine shoots a missile, it takes some time to reload.