A plug (10) for blocking off mining exploration drill holes. The plug (10) comprises an elongate body (12) of solid impervious material, and an engineered surface (14) is provided on an external circumference of the elongate body (12). The engineered surface (14) comprises an annular anchoring groove (16) having a ramp (18) extending from a maximum diameter to a minimum diameter of the engineered surface (14) in a braking direction, in which it is intended to prevent movement of the plug (10) in the drill hole. The plug (10) also comprises an O-ring (20) received in the anchoring groove (16) and having a thickness whereby it protrudes slightly above the maximum diameter of the engineered surface (14) and engages with a wall (22) of the drill hole. In use, when the plug (10) is moved in the braking direction in the drill hole, the O-ring (20) rolls up the ramp (18) and wedges between the engineered surface (14) and the wall (22) of the drill hole to provide a braking action for the plug (10) against the wall (22) of the drill hole.