The pilots of a flight that dropped 21,000 feet in 10 minutes are being investigated over claims they were smoking in the cockpit at the time, sparking the incident.
The oxygen masks on Air China flight CA106 from Hong Kong to Dalian, China, were deployed approximately half an hour into the Tuesday night trip when it began to lose altitude.
Data from flight tracking site FlightAware shows the plane falling from 35,000 feet to 14,000 feet before levelling off.
Passengers shared images from on-board the plane onto local blogging site Weibo, showing what appears to be a relatively calm cabin.
One flyer said of the incident: "The announcement from the cockpit said the ability to increase oxygen in the cabin malfunctioned so the plane lost pressure."
The plane landed without incident.
According to local reports, there are suspicions the crew caused the incident by smoking in the cockpit, resulting in the plane losing pressure and, as a result, altitude.
The airline has vowed for "zero tolerance" if crew members are found guilty of wrongdoing.
It has cancelled some scheduled flights following the incident.