Royal Society expeditions The purpose of the expedition was to attempt to settle the question as to the formation of coral atolls. The first expedition experienced defects in the boring machinery and the bore penetrated only slightly more than 100 feet (approx. 31 m). The second expedition reached a depth of 557 feet (170 m). The third expedition was successful in carrying the bore to 1,114 feet (340 m). The results provided support for Charles Darwin's theory of subsidence; in which there are four stages in development of coral reefs: a volcanic island forms, is surrounded by a fringing coral reef, as it subsides slowly a wide barrier reef forms, then after it has sunk below sea level the coral continues to grow forming a circular atoll. However, the geologic history of atolls is more complex than Darwin (1842) and Davis (1920 & 1928) envisioned. Structure of Funafuti atoll The average depth in the Funafuti lagoon (Te Namo) is about 20 fathoms (36.5 metres or 120 feet). See also ISO 3166-2:TV List of mammals of Tuvalu List of birds of Tuvalu List of butterflies of Tuvalu Funafuti Conservation Area Geography of Tuvalu References Subdivisions of Tuvalu Tuvalu, Islands Tuvalu 1 Tuvalu Tuvalu-related lists