Catching turtles

My SO's family owns land along the Mississippi River and her grandfather told me today he believes there are some huge loggerheads in the pits on his land. Anyone know how people catch really big turtles? I saw some turtle traps online but they look like they would only be good for little turtles. When I was a kid my dad would take me in ditches where he saw big turtles and we would poke in the mud with a solid stick to find them but I'm not doing that in these big pits.