Six drown in China well, one after another
Six members of one family drowned in a well in south China after one man fell in and a bizarre series of rescue attempts went disastrously wrong, Xinhua news agency said on Sunday.
The man fell into the well in a village in Guagdong province on Friday evening while he was installing a water pump, the agency said.
"One male relative jumped into the well trying to save him, but without any success, and didn't come back," the agency said.
"Four other men jumped into the well, one by one, trying to save their relatives, but in vain. All were drowned."
Ref. https://www.alertnet.org/thenews/newsdesk/PEK273276.htm
I think there is no mystery and there is a logical explanation to what happened. The carbon bioxide (CO2) is heavier that the oxygen (O2), so at the bottom of the well there was an accumulation of CO2. Most likely they were not aware of this fact and they died from lack of air (oxygen).