How the electricity from the lightning
entered the sealed area
is still under investigation:

Family members of miners killed by an explosion at the Sago Mine in January say a state report on the tragedy leaves too many questions unanswered...
The report also says the miners' emergency air packs "did not perform in the manner expected,"

CNN 11.Dec.06
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Various ways of expressing the same thing:

Lightning is official reason for Sago Mine explosion

Sago Mine explosion caused by lightning

State to blame lightning in Sago

Sago Mine Explosion Caused by Lightning

Investigators say lightning caused fatal Sago Mine blast

Sago Operator Cited for Flawed Breathing Gear
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Sago previously here here, and here