Honestly? I'm not sure whose idea it was. I'd like to take credit for it, but the odds are that someone just lost a baseball out in that field and, while everyone was crawling around on their hands and knees, decided "Hey, this is kind of fun!"
I refer, of course, to the extensive system of hallways and rooms created in the tall grass in our pasture; an area so vast and so riddled with passageways and rooms, we kids got lost.
About a year or so after we moved in, Dad's prairie grass came up in the front pasture. This wasn't really all that exciting until it grew taller than me, a feat which is all the more impressive because my 12-year-old self was currently engaged in yet another growth spurt. (It's only recently that I quit changing pant sizes two or three times every summer, and thank goodness too. Any taller and I would start clocking my head on every single doorframe.)
As kids, of course, my siblings and I did what came naturally; build forts in it. This was accomplished by carefully flattening the grass into the ground, creating rooms and hallways with VERY comfortable floors and walls taller than we were. It was possible, of course, to jump high enough to see over the grass, but that was avoided at all costs. I'd imagine that it would be amusing to see the religiousness with which we refused to just break through "walls" of our rooms and hallways every time we got lost; we always followed the pre-existing routes when we were not in construction mode.
Sometimes, we played games out there. As I recall, pretending to be mice was our favorite, especially after I saw a very confused mouse regarding us quizzically from the undergrowth. I made all kinds of new hallways trying to catch him, but he got away. Regardless, we scuttled around on our hands and knees until they were so permanently green that Mom just about gave up on trying to remove the stains. This led to a new game where we all pretended to be frogs...
That was also the summer where I received a cork gun. It was a pretty basic piece wood shaped like a gun, with a piece of pipe over the barrel that one extended, loaded a cork, and than yanked back on hard to fire the cork straight into little siblings' foreheads. I'm not sure who gave it to me. I doubt it was my parents; I give them more credit for caution than that. To be fair, it did come with a cork fastened securely by way of a string to the gun so that it couldn't fly further than a foot. On the other hand, it took me about three seconds to remove said string, and another ten minutes to modify the rubber seal for a tighter fit to the pipe. When I was completed, I had an awesome gun that could kill grasshoppers from about a foot away. (It wasn't accurate at any other distance, but it went quite a ways.)
Anyway, my brother Nemesis would load up on rubber bands, Quill would grab a small compound bow, and I would bring my gun out to the pasture, where we spent happy afternoons playing as bandits or robbers. My sister Squirrel joined us, but she preferred to load up with 4th of July parade candy--she had an overactive sweet tooth and a detest for violence, even pretend violence. She was usually our "cook."
Dad's dream was to be able to cut all the prairie grass down in the fall and bale it for hay. Unfortunately, that didn't happen, so he was forced to purchase hay bales. We kids helped him unload it and stack it neatly in the barn, then gleefully rearranged it into a fort the second his back was turned. A few weeks later, he restacked all the bales by himself. Nothing daunted, we destroyed that pile too and built a cave. Fortunately for Dad's sanity, it snowed soon after, and we were all so busy building caves and forts in the huge drifts that covered our farm (yay hills!) that we didn't even notice when he took apart our haystack cavern and restacked the bales again.
...I'm starting to realize why Mom banned us from the house during all summers. We were destructive little squirts...