The Major League Baseball rulebook states that bases must be white, 15 by 15 inches, covered in canvas or rubber, 3 to 5 inches tall, filled with soft material and attached to the ground. Bases are a few inches tall around the perimeter and domes slightly to a maximum height. They are heavy, filled with a thick rubber and covered with a synthetic, striated white coating that does not puncture under the weight of metal spikes. The middle of the underside contains a protruding metal nub that interlocks with a permanent metal-lined hole on the field.
The rubber on a base has very little give, feeling more like hard vinyl. Moreover, the bases don't move once anchored to the ground. This sets up runners for ankle sprains, knee injuries, torn ligaments, dislocated fingers and shoulders, and broken bones if they make contact with the base awkwardly..