How do I unclog a frozen underground drain

Put straw on the ground about 1 foot thick and 4 feet wide and it will thaw but will take some time.
Now if the actual Drain is under the ground then some one buried is and shouldn't have. If the underground "drain" is the sewer waste line "drain line the go to the above.
You could also try rooting in out with the auger and try to grind up the ice to get a hole and then run hot water to open the rest of the hole bigger.

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