500 Foot Long Crack Forms in Michigan - No Explanation

Eileen Heider was relaxing in her recliner when she began to feel the chair vibrate.  An explosion was heard and when she ran out to check on it she noticed a large crack had formed along her property.

Heider and neighbors heard an explosion-like sound Monday morning. Heider found the nearly 200-yard-long crevice, which measures five-feet deep in parts, Tuesday. (source)

So far, a gas eruption has been ruled out, but nothing else developed on this odd story all week.  There's no fault line nearby, and no history of geological activity like this.

Here's a video, wherein a local man says his large pile of wood was knocked over in the explosion:



Since there's no crater, we can probably rule out ordinary explosives.  No one has located an epicenter, yet, so we've got nothing to go on.  Could there be a geyser or a volcano forming?  That's pretty rare, but so is this incident.  Also, there's a situation called Bradyseism in which the ground shudders like this due to underground magma fluctuations.

There's also the liquification phenomenon (which has been mentioned on some comment boards) where the ground suddenly acts like a liquid and sucks things up.  Supposedly, this sort of thing happened a long time ago on an island inhabited with pirates, and many of the criminals on the island were trapped in the ground after it hardened and became meals for local animals.  It's very creepy, but I don't see anyone describing that sort of thing, and it's a phenomenon that is usually observed in loose sandy soils - not clumpy moist soil with vegetation.  Also, liquification isn't known to cause audible explosions.

But this sort of speculation is all we have, right now.  Maybe we'll get some more answers, or maybe this mysterious crack in the ground will always haunt us.