SCP-173 is made of rebar and concrete. If you don't look at him at all you're dead. He snaps necks if you don't look at him. His speed is unknown as one moment he's there then he's gone when you blink but you're most likely dead if you do. If the area is too dark and you can't see him properly then you're dead. If you get too close and don't blink and stare at him he'll snap your neck.