World's Only Moving Mud Puddle

Gas goes up, mixed with fluid, after it passes the soil, the soil settles in a much more compact way, leaving
the indented ground locally. The movement is just a result of it choosing the non-compacted soil, which is then
more permeable to the fluid/gas mixture.
My solution would be using a somewhat bigger pit around it, use floaters for support of the trains tracks and
road, by filling in the now larger pit with more water, which may move but ultimately is settled
in height by a pump either pumping in more water or removing some to maintain a constant water height.

Now Mr. MudPit can go it's way and continue to compact soil, while passing through, and when it's gone
far enough, you can remove the pump and the water and simply fill in the pit again using the same
method as the mudpool, spraying in mud mixed with air.