China Eastern Airlines’ Flight MU-5735 left Kunming, the capital of Yunnan Province, at 1:11 p.m. on Monday for what should have been an hour-and-a-half flight east to Guangzhou, a major commercial city in southeastern China.

After an hour, though, the flight turned horribly wrong, according to data from Flightradar24, a tracking platform.

Recomnded Reads:-#15 Carl Thomas Dean’s biography and know amazing facts

About 2:20 p.m., it “suddenly started to lose altitude very fast,” Flightradar24 said in a tweet.

The plane was cruising at an altitude of 29,100 feet when, in just over a minute, it lost more than 21,000 feet. It appeared to briefly regain altitude around 8,000 feet before continuing its plunge, according to Flightradar24’s data.

The plane was in the far east of the Guangxi region, where weather reports don’t suggest any possible contributing factors. Temperatures reached a high of 86 degrees Fahrenheit around 2 p.m., according to the China Meteorological Administration. Winds were moderate at less than 12 miles per hour, and visibility was 10 miles. Rain was forecast for the evening, but no precipitation had been measured at the time of the crash.