▲ An investigation team is closely examining the newly-discovered lake within Yongcheon Cave. It is believed, though still unknown, that the lake leads to the ocean. Photos courtesy Jeju Special Self-Governing Province
Hidden within the Geomun Oreum (parallel volcanic cone) lava tube system on the Northeast of the island is Yongcheon Cave. It was discovered accidentally back in 2005 and has since yielded a large number of interesting geological and biological discoveries.
Only a small percentage of the roughly 3.6 kilometer-long cave has been studied, and it has yet to be opened to the public.
Yongcheon’s geological importance and beauty have made it stand out, even as part of the illustrious so-called UNESCO “triple crown.” The Geomun lava tube system is a World Natural Heritage Site (along with Seongsan Ilchulbong tuff cone, and Hallasan Natural Reserve), part of Jeju’s biosphere reserve, and a designated Geopark site. The Korean government named Yongcheon Cave, Natural Monument No. 466.
Referring to an ongoing research project, part of a comprehensive scientific study of Geomun Oreum’s lava tubes, volcanologist Jeon Yong Moon said, “The aims of this research are to find traces of lava which erupted from Geomun Oreum and to investigate the formation process of underground caves.”
“There are ongoing archaeological investigations on various historical relics found inside the Yongcheon cave.”
Many researchers, including Dr. Jeon, believe that Jeju Island would not have made the 2007 UNESCO World Natural Heritage list if it hadn’t been for the scientific importance of the cave. Among just some of its notable features are that it includes an approximately 800-meter-long underground lake, and there is evidence that people have used the cave since at least the 5th century.
“There are various kinds of relics, such as earthenware, ironware, shells and animal bones,” Dr. Jeon told The Jeju Weekly in an email interview. “Most relics that were collected are stored in preservation by the National Jeju Museum. Investigations on the ages of the animal bones and the shells are taking place now.”
He added that the relics will eventually be put on display.
Geologically, Yongcheon has some very interesting chemical and physical properties. The most obvious is that the walls of the cave are white due to lime leeching. This sets it apart from the basalt black volcanic rock of the rest of the lava tube system. All the speleothems (cave formations) are very well preserved.
“We call this type of cave a lime-decorated lava tube cave,” said Dr. Woo Kyung Sik, a professor in the Department of Geology at Kangwon National University and director of the Cave Research Institute of Korea.
“There are white sands on beaches around Jeju Island (especially on the Eastern and Western side of the island). These white sands are ... carbonate sediments [which] are the sediments of small skeletons of organisms which secrete their shells with calcium carbonate.”
Dr. Woo explained that carbonate sands are mostly made out of mollusks, calcareous red algae, and benthic foraminifera. These sands leeched through to the lava tube caves to form carbonate speleothems less than 6,000 years ago, when the sea level rose due to the melting of glaciers in the last glacial period.
Yongcheon’s stalactites — formations which hang down from the ceiling — are generally 1.5 meters or more in length. It takes on average about 100 years for a stalactite to grow 3 millimeters.
Dr. Woo has conducted an independent paleoclimatic investigation in Yongcheon Cave using two stalagmites — formations which grow up from the cave floor due to the dripping of mineralized solutions —one 1,400 years old and the other 400 years old.
“We got a pretty good record out of Yongcheon Cave,” Dr. Woo said, adding that the data will be published later this year or early next year.
Another major scientific discovery was an approximately 800-meter-long lake and the 2010 finding of a previously unknown fish found living in the lake.
“The formation process of the lake in Yongcheon Cave is very simple. Before the sea level rise, it was a normal lava tube cave. That's why you can find common morphological features and lava speleothems in the submerged part of the Yongcheon Cave. After the sea level rose due to melting of glaciers, the lake was formed,” Dr. Woo said.
Describing the chemistry of the lake, he said, “Because the lake was connected to the sea through the very small hole (we are suspecting this), the chemical composition of the bottom part of the cave is very close to that of seawater whereas the shallower part is more fresh. After heavy rain, rainwater penetrates into the cave through cracks and has supplied more water into the lake. Thus, the chemical composition of the upper part of the lake is less saline than that of the lower part.”
Non-microbial life has found its way into even such a remote and unlikely place as the underground lake.
“As the sea level rose, sea organisms were washed into the cave along with the seawater,” Dr. Jeon explained. “The seawater also carried sediments, and it seems that these sediments gradually piled up and blocked the cave.”
“I believe that the living organisms went through unique evolution in this isolated environment, which was different from any other environments the creatures had lived in.”
The as-yet unidentified fish found by a documentary team during filming deep in the cave are described as living on the sandy bottom and measuring between 4 and 7 centimeters in length. The fish are blind and appear to be similar to gobies seen around the Jeju coast. This type of fish has never before been found in a cave environment in Korea.
According to a government report on the discovery, Dr. Kim Byung Jik, a scientist from the Bio-Resource Center in the Ministry of Environment, said, “When considering the fact that the Yongcheon-donggul cave lake has been isolated for a long time, the discovered fish species are highly likely a new kind that has gone through the process of degenerative evolution inside the cave.”
Regarding this discovery, Dr. Woo remains cautiously optimistic.
“KBS [TV] reported blind fish in the lake last year. ... but we do not have much information,” he said, adding that there is much about Yongcheon Cave still to be studied. “We even do not know the exact length of this submerged part.”
(Translation by Koh Yu Kyung)
ⓒ Jeju Weekly 2009 (http://www.jejuweekly.com)
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