At least 91 people were missing on Monday after a massive landslide in China, reports indicate.

According to Xinhua, China’s official news agency, about 3,000 rescue workers were searching through an industrial park in the city of Shenzhen after construction waste slid down a hill at about 11:40 a.m. Sunday, covering a 93-acre area in 32 feet of dirt. The slide reportedly affected 33 buildings, either burying them or otherwise damaging them, and hospitalized 16 people.

Xinhua reported authorities detected some signs of life underneath the landslide, but the muddy composition of the silt was complicating rescue efforts. While 900 people were safely evacuated after the disaster, the news agency reported only seven people had been rescued from the mud.

A researcher with the China Academy of Railway Sciences, who was assisting the rescue, told Xinhua it was the only time he’d seen a landslide of this magnitude in his 30 years on the job.

Xinhua reported that the landslide caused part of an important pipeline that carried natural gas to nearby Hong Kong to explode. Crews were reportedly building a temporary pipe on Monday.

It’s still not known what exactly spurred the landslide, according to Xinhua. It reportedly occurred at a former quarry that had been turned into a site where construction waste could be dumped.