Clay soil. The bubbles aren’t flammable, I held a long lighter over them and they actually blew it out. I got close and smelled the bubbles, they didn’t smell distinctive, maybe slightly sewery. There are no utilities that I know of under there, but the area has been occupied for a long time, and my house is 100 years old.
The water is slowly draining into an airtight cavity underground and replacing air in the cavity, which is bubbling up through the same hole. The cavity is probably why you have a puddle there to begin with, the ground is depressed into the cavity. It could end up forming a small sinkhole. Excavation recommended.
Makes sense. Thanks