A coal mine accident that happened on Sunday in central China’s Hunan Province has killed 18 people, local authorities said Monday morning.

A gas leaking occurred at the Jilinqiao colliery in Huangfengqiao Township, Youxian County, around 11 a.m. Sunday, when a total of 55 workers were working in the mining shaft.

Rescuers have brought 37 miners to safety and sent them for hospital treatment.

Investigators are still testing substances in the poisonous gas. Police have detained those responsible for accident pending investigation.

Chinese researchers are using artificial breeding to control the population of captive-bred Siberian Tigers and improve their genetic quality.

There are about 1,000 Siberian tigers at China Hengdaohezi Feline Breeding Center, the world’s largest breeding center for tigers in captivity. Researchers control the number of newborn cubs to about 100 every year.

“Not all adult Siberian tigers are fit for reproducing. If we do not control reproduction, the quality of the offspring will be affected,” said Liu Dan, chief engineer at the Siberian Tiger Park.

Researchers conduct DNA tests to select parents and to prevent inbreeding, he said.

Most Siberian tigers have more than one cub at a time. It is common for a mother tiger to give birth to three cubs at once, Liu said.

The oldest mother at the center is 15 years old. Some tigers can reproduce from the age of four until age 14, he said.

Researchers have also given the animals wildlife training.

“Wildlife training is an important way to maintain their quality. I hope one day we can release these captive-bred animals to the wild,” he said.

Southwest China’s Tibet Autonomous Region has banned visitors from passing through a state nature reserve to access other areas, as an environmental protection measure.

In a circular released Friday, the regional forestry bureau said that any crossing at the Changtang National Nature Reserve (CNNR) will be illegal. It specifically mentions that people should not pass through the CNNR to reach two other state nature reserves, one in Xinjiang and another in Qinghai.

The CNNR covers six counties in Tibet’s Nagqu Prefecture. With an area of 298,000 square km and an average altitude of 5,000 meters, it is China’s biggest and highest reserve. The area is a wildlife paradise, and home to a variety of wildlife species and numerous lakes.

The circular called on tourists, adventure enthusiasts and tourism agencies to comply with the reserve’s laws and regulations to “protect the last pure land on earth.”

Tibet has increased protection efforts at the CNNR. In 2015, the regional government established 73 management stations at the reserve and hired a total of 780 farmers and herdsmen to patrol, manage and protect the reserve at a cost of 300 million yuan (43 million dollars).

Police in southwest China’s Yunnan Province have busted a drug-trafficking case, with three suspects detained, authorities said Saturday.

More than 57 kg of methamphetamine and two vehicles were confiscated in a raid on Tuesday, according to the public security bureau of Zhenyuan County.

On Tuesday, police intercepted an SUV on a local highway in Zhenyuan and detained a suspect surnamed Xie. Later, another SUV carrying another two suspects was stopped.

Further investigation is under way.

The ruins of an ancient city have been discovered in southwest China.

Archaeologists have found the remains of several walls in Yunnan’s Midu County, believed to be part of Baiya City, and dating back to the early years of the Tang Dynasty (618-907) more than 1,000 years ago.

Accounts of the old town of Baiya City first appear in a history book written by military official Fan Chuo during the Tang Dynasty. Fan said that Baiya City had an old town and a new town, which were closely connected. A previously discovered “old town” was about two kilometers from the confirmed new town, contradictory to the book, according to historian and archeologist He Jinlong.

“From the location of these newly discovered ruins, much closer to the new town, we are pretty confident that it is the authentic old town,” He said. “As excavation continues, we will know more details.”

Domestic and foreign records about Baiya City are quite limited, and the new discovery will provide valuable evidence for research into the ancient civilization of west Yunnan, He added.