Sanmenhai

Sanmenhai is a cave in the Leye-Fengshan geopark in Guangxi, China. The cave is unique in that it features seven skylights that mimic the layout of the constellation Ursa Major (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. It is an outlet of the Poxin underground river, which formed during the Permian 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 2.4 km underground river into southeast and northwest sections. The southeast portion is accessible by tourists, and extends 0.7 km 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 50 m and 70 m, respectively.

The Poxin underground river undergoes daily tidal fluctuations, rising by 20 cm at night.

Sanmenhai is a typical karst landform. In an area of less than 1 sqkm, 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 107 m in the east–west direction and 98 m in the north–south direction, with a depth of 72 m. 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 85 x. The bottom of the skylight is half-covered by water, at a depth of 19 m 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 75 m, and a diameter at the water level of 43 m. The water depth is as much as 22 m with an extra 10 m 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 30 x and water as deep as 70 m. The total height of this skylight (including the depth of the water) is 118 m.