Apo Reef two lagoon systems, the north and south lagoons which are bounded by narrow reef platforms. It is a almost triangular northern and southern atoll-like reefs separated by a deep channel that is open to the west. The channel runs east to west from deep with a fine white sand bottom, numerous mounds and patches of branching corals under the deep blue water. The north lagoon is an enclosed triangular coral reef platform partly exposed during low tide. It is relatively shallow with depths of about . While the south lagoon is an inverted triangular coral platform enclosed on two sides
Coral Sea Islands and 1880s the islands were mined for guano but the absence of a reliable supply of fresh water prevented long-term habitation. The Coral Sea Islands became an Australian external territory in 1969 by the "Coral Sea Islands Act" (prior to that, the area was considered part of Queensland) and extended in 1997 to include Elizabeth Reef and Middleton Reef nearly 800 km further south, already in the Tasman Sea. The two latter reefs are much closer to Lord Howe Island, New South Wales, (about ) than to the southernmost island of the rest of the territory, Cato Island. The islands,