Complex sentence: The formation of coral atolls was a scientific puzzle, as in 1824 and 1825 Quoy and Gaimard observed that the coral organisms lived at relatively shallow depths, but the islands appeared in deep oceans.
Split into simpler sentences: The formation of coral atolls was a scientific puzzle at that time. In 1824 and 1825, Quoy and Gaimard observed that the coral organisms lived at relatively shallow depths, but the islands appeared in deep oceans.