How about using an atomic bomb, I wonder if 20 kilotons would be enough? I know back in the 1950’s, it was proposed both in the US and USSR and atom bombs can be used for engineering purposes such as digging big holes and so on, maybe we can use one to cause a ruckus down there to the point where it would liberate so much dirt and rocks, it would seal the hole.
I doubt it needs to be that big.
Particularly if they could drill a hole near the blowout shaft and pack it will explosives.
Make that “with explosives”...