Joy Global Inc., the world’s biggest manufacturer of underground mining equipment, fell the most in six years after cutting its forecast for 2015 earnings and revenue amid a global commodity downturn. Joy tumbled 16 percent to $18.64 at 10:23 a.m., after earlier dropping as much as 20 percent, the most intraday since November 2008. It was the biggest decline today in the Standard & Poor’s 500 Index. Joy’s shares have plunged 60 percent this year as struggling miners buy less machinery. More from Bloomberg.com: Welcome... More