In pictures: Liberia: Rubber republic

There is a real art to tapping the rubber trees.

"The bark is the company's most precious asset," says Ballah Sumo, who has worked at Firestone for 26 years.

He says the tappers only cut 1/16 of an inch of bark at a time.

They start cutting the tree exactly 68 inches from the ground and go down at a 30 degree angle.