The islands are of volcanic origin with basement rocks forming between the late
Cretaceous and early
Eocene. Tectonic movements raised the seabed to allow coral building. The whole Rennell area is thought to have been initially deposited as coralline algal limestone and then dolomitized. This dolomitic reef complex is overlain by younger undolotomized reef limestone. The island is the second largest upraised coral atoll in the world. It is largely unmodified and includes the largest lake in the South Pacific. The island is about one hour's flight in a twin engine prop aircraft south of Honiara. It is almost totally surrounded by cliffs, with the eastern end dominated by a large lake, while the western end is relatively flat with rolling forested hills. One road, known as the Copperhead Road, traverses the length of the island from the Tigoa airstrip in the west to the Labagu port area in the south, with a branch heading east towards the lake.
Kangava Bay has beautiful white sands beach and coral reef which makes it good for snorkeling.
Lake Tegano Official Solomon Island maps name this Lake Te Nggano, while locals refer to the lake as Tegano or Tungano or Big Water. The lake is in length and wide, with an area of , which occupies 17.6% of the total area of Rennell Island. The lake is located at
East Rennell, the southern portion of Rennell Island, in the central basin which was the old lagoon. The hard bottom is covered by several meters of suspended mud. The lake has an elevated salt concentration being kept by a subterranean duct system which connects it with the sea. The water depth is and consists of a mixture of
brackish fresh and salt water. The average distance from the lake shore to the ocean is with wildlife predominantly
eels and water snakes and a bird breeding area. The lake is listed as a
World Heritage Site. On the easternmost point of the lake is the former World War 2 airbase Tigoa. Information from locals claims that there are nine aircraft at the bottom of the lake, with five visible from a boat. Lake Te Nggano was used as a base for flying boats by both Japanese and American forces during the
Pacific War (World War II). Japanese
Nakajima A6M2-N Rufe floatplanes operated out of Rennell Island until US air and sea superiority made the base untenable. American
PBY Catalinas used the lake as a base after 1943. A detachment from the 2nd Marines was sent from New Hebrides and arrived at Lake Tegano on 12 November 1942 to establish a lookout post. The US forces scuttled eight of the warhorses at the end of hostilities rather than take them home. Reports were made of crash landings due to the coral outcrops within the lake. Members of the local community attempted to retrieve one of the radial engines of the aircraft to use as a generator using only man-power. First it was cut from the wing by diving with
snorkels and using hand-tools. It was then dragged across the coralline rock lake floor by hand-winch. They were overcome in their task by the engine's tremendous weight. They did manage to get it close to the shoreline before giving up, close enough that one of the propeller blades is exposed to the air. Very quickly however, the engine became unusable through corrosion, so further attempts to bring it ashore were abandoned. the endemic orchid
Dendrobium rennellii near Lake Tegano, two endemic species of
Pandanus (
P. lacustris and
P. rennellensis).[] There are also 11 species of bats in the area including the
Rennell flying fox (
Pteropus rennelli) which is endemic to the island. Lake Tegano is the only known location for the endemic
sea krait Laticauda crockeri (VU), one of only two known freshwater sea snake species in the world. The other species of sea snake in the lake is
Laticauda colubrina. There are five species of geckos, four skinks, the
Rennell monitor (
Varanus juxtindicus) and three snakes, all of which are species with widespread distributions and are typical of the region. There are 27 species of land snails, seven of which are endemic to the island,
coconut crab (
Birgus latro) (DD) and two other species of land hermit crabs (
Coenobita spp.). A total of 731 insects have been identified from collections made at Rennell and Bellona. Moths (Lepidoptera) have the greatest number of species (246 in total) with 35 species and 25 subspecies exclusive to Rennell and Bellona. Renbel has no indigenous malaria, cane toads, vipers or crocodiles. ==Culture==