Who says we're done exploring the Earth?

Scientists in China announced a major new discovery this week: 49 massive sinkholes that were previously unknown, representing the highest natural density of the phenomenon in the world.

Government researchers discovered the sinkholes during the course of a four-month survey in the Qinling-Bashan Mountains, which are located in the Hanzhong area of northwest China's Shaanxi Province.

The 49 sinkholes are clustered in a 230-square-mile patch of land within the 2,000 square miles that were surveyed.

The largest of the sinkholes has a diameter of 1,706 feet and a depth of 1,050 feet.

The scientists also found several species of rare plants and Chinese giant flying squirrels, which have striking red fur.

The team suggests the holes likely formed slowly over hundreds of thousands of years as underground water dissolved carbonate rock, such as limestone, under the surface. It's the same process that is frequently seen in other places around the globe—irregular landscapes commonly known as karst areas.

French cave explorer Jean Poutasi inspected one of the newly discovered sinkholes and called it "the world's most beautiful sinkhole," according to Chinese media.

Local officials hope that public interest in the sinkholes, along with the rugged beauty of the surrounding landscape, may attract tourists to the region. They also say they are starting to work on granting protection to the sites, which might contain geological records of past climates.

Sinkholes can open up gradually or rapidly, sometimes swallowing buildings or vehicles and even causing deaths. Without advanced sensing techniques like ground-penetrating radar, sinkholes often leave little indication on the surface as they form, until the soil above them collapses. Occasionally, however, they are preceded by cracks or slumping.

Sometimes landslides are mistaken for sinkholes, but a true natural sinkhole requires that a void form from below. Certain human activity—such as construction or poor water management—can also cause artificial sinkholes, sometimes to devastating effect.