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Majuro and Arno Atolls appear as meandering lines of delicate white against a darker ocean background in this satellite image. Areas of shallow water near the shore appear iridescent blue, especially in the southwest portion of each atoll and off the northwest tip of Arno Atoll. Overhead, tiny white clouds dot the sky. Majuro Atoll is 40 km (25 mi) in circumference, and it partially encloses a lagoon of nearly 300 sq km (115 sq mi). The lagoon is far bigger than the atoll itself, whose land area is less than 10 sq km (4 sq mi); walking from the lagoon side to the ocean side of the atoll takes only minutes. Neighboring Arno Atoll encloses three lagoons: two "pinched lagoons" in the northeast and northwest, and a main lagoon of nearly 340 sq km (130 sq mi). Arno's total land area is just 13 sq km (5 sq mi). Scientists have long understood that coral atolls develop around volcanic islands and remain there after those islands have eroded away. To what extent the final shape of an atoll is influenced by wind and ocean wave activity, however, remains a topic of debate. Image courtesy of NASA.