The cave formations in China's southwest Guangxi Zhuang region almost look like a different world.

Centuries of dissolving limestone rock, a geological process known as karst, have resulted in expansive caverns, with stone towers and massive sinkholes.

During their most recent expedition exploring the caves, Chinese and French cavers spotted an extremely rare fish called a Sinocyclocheilus grahami, or golden-line barbel. According to the International Union for the Conservation of Nature, the species is critically endangered.

The golden-line barbel was once endemic to Lake Dianchi in China's Yunnan province, but now the fish is thought to only survive in about 20 spring ponds and one lake tributary. Invasive species, overfishing, and pollution have caused the species to decline by more