A safety shield !

Here you can see a unit of three shields as they are commonly used in today's longwall mining areas. These units are pushed against the roof with several tons of pressure by the (in this picture) shining hydraulic cylinders and by this they avoid that the miners are endangered by falling rocks etc..

You can see the face chain in front of the shields, as well, and above it there are the black metal "teeth" the shearer uses to move along the face.