He is never more successful than when he comes to men in their idle hours. But his wife looked back from behind him, and she became a pillar of salt. Again the solemn command was given to hasten, for the fiery storm would be delayed but little longer. But one of the fugitives ventured to cast a look backward to the doomed city, and she became a monument of God's judgment. Note that Lot's wife became a pillar of salt before God rained down the fire and brimstone.
Eternal fire means burning until consumed.
There is another time that is still to come, when this whole earth will be cleansed with fire and brimstone and there will be nothing left but ashes under the soles of your feet.