Extract from GIA Textbook: Diamonds

Did you know? -- In an open-pit mine, levels form circular rings around an ever-deepening center. The levels are arranged in a spiral to allow trucks to drive in and out of the central mining area. This is South Africa’s Finsch mine, which was converted to an underground operation when the open pit reached a depth of 1,410ft. (430m). 
Courtesy: GIA. Resource: Amanda Koo