I did not know this was a thing but it does not get more grassroots. Rock crawling mowers

Rock Crawling Mowers in a Hurricane Flood - YouTube

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Respect.

Seems to be a lot like 3 wheel atv trikes except they have 4 wheels

I want this on the upcoming Speedvision.

My back hurts after mowing my yard too fast on my rider. I wouldnt be able to move after that, but its looks super fun. 

I'm not used to seeing mowers with horizontal shaft engines. Is that a particular type of mower or a modification? I guess it's a Lawn Tractor?

Vintage garden tractors like these ones pretty much all had horizontal shaft motors.  Lighter duty lawn tractors more often use vertical shaft motors.

I remember one time as a kid I was fooling around doing some 'off roading' on my dad's Allis-Chalmers lawn tractor, and I flipped the thing going up a hill...it wasn't that hard to do. I was waiting for one of the guys in the video to end up on his back in the middle of that stream.

i'm genuinely impressed with the way these lunatics manage to balance these things on the back axle like that.  i'd never expect them to have that kind of traction!

I was raised using an old Wheel Horse like that. 
 

I never drove it up a stream bed, but with the governor removed  it was easy to ride a wheelie for several blocks