Well I filled with quikrete the best I could, got 4, 80lb bags in there. I filled from the creek side. I could not get it to go all the way through, but I know the hole is all the way through but it isn't straight and gets narrow then larger. I got it in there a good 3 feet.

I think the next plan on this tunnel is to rent a 2 man auger and drill a good size hole down from the top and fill in with quikcrete. Since there is quikcrete on the creek side it will run towards the pond and hopefully completely fill the cavity. I'll have to wait a few days to do this.