A Mysterious Area

There's a mysterious covered area in the very back of my yard.  I've eyed it for quite some time, but because of snow and ice coupled with my natural tendency toward clumsiness, I hadn't been able to actually peer inside.  So I simply stared at it, letting my imagination run wild.

Staring at a covered area that's easily large enough to fit a grown human body is probably not the best course of action for somebody with a very active imagination, but since the weather refused to give me the opportunity to discover the mysteries within it, I was left with nothing but my own ruthless imagination.

What could be there?  Is it a ruggedly put-together storm shelter?  This is southern Minnesota, after all, and the area is set up at the top of a hill.  Perhaps it tunnels into the earth.  And if it tunnels into the earth...

Perhaps there are creatures living inside.  A rabbit warren?  Perhaps a squirrel has taken up residence.  Or even worse... a skunk could live inside.

Ok, I know there isn't a skunk.  If there was I'd have smelled the tell-tale signs.  I can cross that off my list.

So what exactly is there??? 

Today is warm, so I'm heading out to see.  More on this later...