How deep do fire ant colonies go down?

It isn't so much the depth, but rather the distance the tunnels go. Not uncommon for one "colony" to have multiple openings several hundred feet apart. That's why they are so hard to kill. Kill this end the other end still fluorishes.

I never have dug a colony up but have been told by my local agriculture agent that 3 to 4 feet deep is not uncommon.

One researcher found some tunnels go down 10 ft.

Depends on the soil, but 3 to 4 ft should be no problem.

that will depend on the weather!! when it rains they close off the top of the nest and go deeper,, if the rains continue to soak in they go deeper!! during drier climes they will move to the top to cool their eggs and young,, if its too hot they will find the perfect level and cool the nest in several ways

twenty three kilometres is the deepest an ant has been found.