We are in lock step on the culvert idea! I was also looking at making it a root cellar/tornado shelter. Our property is in the tornado alley. Be a nice place to jump into in the event of a twisty! And a fan in the back strong enough to suck your hat off!
The nice thing about the concrete culverts is you can add precast box ends to add space in any direction. The above grade bit is so you can never drown in it...no matter what happens.