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The edifice is about 600 km (370 mi) wide. Because the mountain is so large, with complex structure at its edges, allocating a height to the structure is difficult. It stands 21 km above the Mars global datum, and its local relief, from the foot of the cliffs which form its margin to the northwest to its peak, is nearly 22 km (14 mi) (a little over twice the height of Mauna Kea as measured from its base on the ocean floor). The total elevation change from the plains of Amazonis Planitia, over 1000 km to the northwest, to the summit approaches 26 km (16 mi). The summit of the mountain has six nested calderas (collapse craters) forming an irregular depression 60 × 80 km (37 × 50 mi) across and up to 3.2 km (2.0 mi) deep. The volcano's outer edge consists of an escarpment, or cliff, up to 8 km (5.0 mi) tall, a feature unique among the shield volcanoes of Mars. Olympus Mons covers an area approximately the size of Arizona.