cottonwood tree with bog

I have this very nice cottonwood tree in my back yard, at the bottom of tree there is a wee bog. A spot about about 2 feet square that is always boggy. There is no water source anywhere near. Not just damp but standing water. we have been in a very hot dry spell (temps over a 100) and I still have my bog. There is also an exposed root that holds water. no one has an answer.

perhaps its a spring bubbling up to the surface.

Are there any nearby water lines ? Trees can rupture those.

Leach field? Septic tank? Old well?

I have a leach field that feeds some trees. Mine dumps into a French drain that will eventually fill with roots. The pipe is just below the surface and not in standing water. This should delay the intrusion of roots into the pipe. Most systems don't have this safeguard. If your septic system ever backs up, this tree is a prime suspect.

Grey water systems can also feed trees. My friend Ted had a pipe that discharged into a rock pit in his orchard. Every few years, he would lift out some of the stones and use his loppers on any roots in or near the pipe. He also dug out all of the soil to a depth of 18 inches in a one sq. yard area. The system is now about 45 years old and works well.