Ice sections from depth of 71 (top) and 300 metres (bottom) at Byrd Station are magnified 3.25 diameters (a grid square represents one square centimeter). The numerous tiny gray, white and black spherules are air bubbles trapped by the ice. With increased depth, ice crystals tend to become larger and air bubbles smaller. The deeper ice was deposited as anow about 1,600 years ago (Scientific American, Spet. 1962, p. 133).