Employee was carrying bolter, right foot slipped on loose rocks & he fell to the ground, landing on left knee. Reported incident to supervisor. Knee continued to swell, he requested a physician's eval. Saw Dr. Lao on 1/17; MRI requested to determine any soft tissue damage to knee. Released to full duty. 1/31 MRI results indicated meniscus tear. Surgery on 2/03. Reportable on 1/31.