The rest of the vertical column is made from a material called syntactic foam - a solid made mostly of hollow "microballoons" - giving it enough buoyancy to float back up.
Objects which hitherto had to be cast in solid metal can be manufactured as hollow shapes, reducing their weight and the amount of precious metal used.
At a recent golf-industry show, however, the U.S. working group displayed examples of counterfeit clubheads sawed in half, and you can see the type of irregularities that impact performance: irregular interior walls, or supposedly hollow iron-head cavities that instead are solid steel.