I have been meaning to try another method of dealing with gophers. You
could use a two liter bottle to do this. Cut the bottom off the
bottle, insert the top into the gopher hole, and put dry ice is the
bottle. The cool CO2 will flow into the hole and force the oxygen out.

At least that is the theory. I have not tried it. But a good many
years ago a friend, Bill Torode, and I were looking for caves in
Northern Alabama. We found a sink hole with a lot of leaves in the
small entrance. We started in, but our acetylene lights went out when
we were about ten feet into the sink. We found that our light would
not stay lit below some level and figured it was CO2 gas in there. We
moved on and looked elsewhere.

Dan