It's only about two millimeters thick, but it might be the most powerful substance in golf: a highly neutralized co-polymer resin (or, in English, a ball's inner mantle layer).
It's clearly visible when you cut open a multilayer ball—those with urethane covers and those with ionomer covers designed to help average golfers get more distance.
"Along with the improvements in the urethane cover, I think this material is the most improved part of the golf ball in the past 10 years," says Rock Ishii, product-development director for golf balls at Nike Golf.
The beauty of this thin inner layer is that it adapts to the cover's properties, whether it's the soft, spin-inducing urethane-covered balls preferred by tour players or the firm, high-speed ionomer-covered balls designed for average swings. With urethane-covered balls, the inner layer reduces spin on tee shots. On ionomer-covered balls, the inner layer is soft to improve feel and still minimize spin.
Ishii says the inner layer's evolution grew out of necessity. "We were trying to design the ball to optimize launch conditions for each club in the bag," he says. "We discovered by adjusting the inner-layer hardness and thickness we could adjust those launch conditions without negatively affecting the driver or the wedge."
The inner layer is the golf ball's control valve. "In the club-ball impact, the vast majority of energy is lost in the ball," says Steve Ogg, vice president of golf-ball research and development for Callaway. "Think of the mantle as the layer in the ball that helps reduce the amount of deformation that the core sees."
Ogg believes the inner mantle layer is the most intriguing element of golf-ball technology. "It's more complicated than the cover or the core," he says. "You're going to see a lot more varieties in the mantle. That's one area left for design improvements."
The debate on multiple inner layers is split in balls such as the Nike One and Bridgestone's B330-S.
Some believe the shift in hardness from the core to a (firmer) outer mantle results in quick ball restoration at impact for more speed and less spin.
Others say the varied hardnesses are designed to work better with either irons or woods.