I bought a pasture like that once. Beautiful piece of land, good stand of grass, great price, I bit. Same thing as you describe. I eventually found out the whole area had a clay bed like a big bowl with the low spot right under my pasture, which was basically floating. You could drop a post hole digger 2-3' in the heat of summer and hit mud/water. It would get hard on top, and you were mostly ok in the summer. But other times when it looked fine, and would be fine for a bit, tractor tires on my old tractor would break through, and it was straight to the bell with little warning. I was able to winch/walk it out most of the time, but also had to higher a track dozer, and it almost got stuck once. Never saw a big dozer like that look like it was getting stuck before. I was so glad to sell that plot...