Sanmenhai is a cave in the Leye-Fengshan

geopark A geopark is a protected area with internationally significant geology within which sustainable development is sought and which includes tourism, conservation, education and research concerning not just geology but other relevant sciences. In 20 ...

Guangxi Guangxi (; ; Chinese postal romanization, alternately romanized as Kwanghsi; ; za, Gvangjsih, italics=yes), officially the Guangxi Zhuang Autonomous Region (GZAR), is an Autonomous regions of China, autonomous region of the People's Republic ...

China China, officially the People's Republic of China (PRC), is a country in East Asia. It is the world's most populous country, with a population exceeding 1.4 billion, slightly ahead of India. China spans the equivalent of five time zones and ...

. The cave is unique in that it features seven skylights that mimic the layout of the constellation

Ursa Major Ursa Major (; also known as the Great Bear) is a constellation in the northern sky, whose associated mythology likely dates back into prehistory. Its Latin name means "greater (or larger) bear," referring to and contrasting it with nearby Ursa ...

(the Big Dipper).

Etymology

''Sanmenhai'' translates into English as ''three doors accessible to the sea''. The cave is also known as ''Shuiyuandong'', meaning ''cave that produces water''.

Location and geology

Sanmenhai is located in Poxin, a village in

Fengshan County, Guangxi Fengshan County (, zhuang: Fonghsan Yen) is a county of Guangxi, China. It is under the administration of Hechi City. Administrative divisions Fengshan has one town, Fengcheng (), and ten townships: *Paoli (袍里乡) *Zhaiya (砦牙乡) ...

. It is an outlet of the Poxin

underground river A subterranean river is a river that runs wholly or partly beneath the ground surface – one where the riverbed does not represent the surface of the Earth. It is distinct from an aquifer, which may flow like a river but is contained within a per ...

, which formed during the

Permian The Permian ( ) is a geologic period and stratigraphic system which spans 47 million years from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.9 Mya. It is the last period of the Paleoz ...

period.

Exploration

The seven skylights of the cave are designated Skylight I - Skylight VII. Skylights I - IV extend westward from the entrance of the cave. Sanmenhai is delimited by Skylight III, which divides the underground river into southeast and northwest sections. The southeast portion is accessible by tourists, and extends from the outlet to Skylight III. Only three of the seven skylights are accessible by boat on the underground river: Skylights I, II, and III. Skylights IV and V are accessible by diving through the river a distance of and , respectively. The Poxin underground river undergoes daily tidal fluctuations, rising by at night. Sanmenhai is a typical

karst Karst is a topography formed from the dissolution of soluble rocks such as limestone, dolomite, and gypsum. It is characterized by underground drainage systems with sinkholes and caves. It has also been documented for more weathering-resistant ro ...

landform. In an area of less than , in encompasses peaks, sinkholes, natural bridges, and karst lakes. skylight in sanmenhai is also called karst window(tiankeng) by Chinese, it is sinkhole-like formation, but differ from sinkhole

Skylight I

Skylight I is the biggest among the seven skylights. It is roughly circular in shape, with walls that are covered in vegetation. The diameter of the opening is in the east–west direction and in the north–south direction, with a depth of . There are four routes to Skylight I: the upper and lower reaches of the Poxin River; Gandong cave in east and south; and a water cave from underground. In 2009, a diving exploration found that there is no cave channel in underground.

Skylight II

Skylight II is an oval measuring . The bottom of the skylight is half-covered by water, at a depth of below the rim. Some plants grow horizontally on the wall of Skylight II.

Skylight III

Skylight III is 690 m from the outlet. It is funnel-shaped, with a top diameter of , and a diameter at the water level of . The water depth is as much as with an extra of sediment. In 2008, explorers found that there was an underwater cave connecting Skylight III with Skylight IV.

Skylight IV

Skylight IV is also funnel-shaped, with top dimensions of and water as deep as . The total height of this skylight (including the depth of the water) is . http://leyefengshan.cn/

Gallery

File:坡心河出口.jpg, Outlet of Poxin river File:三门海1.jpg, Skylight I File:三门海2.jpg, Skylight II File:天窗3.jpg, Skylight III

References

{{coord missing, Guangxi Caves of Guangxi