A Mine Support is a Support Beam with an 8x tile radius. Support Beams cannot be moved once they have been placed.
Each time a Wall tile is Mined there is a chance to trigger a Cave-In. A Cave-In will scatter new Rubble Walls randomly in areas that you have already Mined out around the source of the Cave-In. Being under a Cave-In when it falls will instantly cause you to become Knocked Out and will damage nearby Structures. The chance of a Cave-In happening is based off the wall's proximity to other Un-Mined Walls, its proximity to already existing Support Beam and your Mines & Mountains Proficiency. While very rare it is technically still possible to get a Cave-In while mining under the radius of a Support Beam. An impending Cave-In will be evident by a rumbling sound and dust falling from the ceiling and can be stopped by building a Support Beam directly under the exact spot where the dust is falling.