Preheat the oven to 225 degrees.
Spread the popped popcorn over a large cake pan or cookie sheet that has sides. Set the pan aside while you prepare the caramel coating. (For easy clean up, you can line your pan with parchment paper)
To begin the coating, use a medium saucepan, or dutch oven, and melt the sugar, butter, and corn syrup on medium to high heat. Be sure to stir it constantly to avoid burning. It will begin to bubble and pull away from the sides of the pan. Continue stirring for about 1-2 more minutes before removing from the stove top. Stir in the baking soda, which will begin to foam in the mixture. Keep stirring until it is well combined.
Pour the mixture evenly over the popcorn and toss the popcorn so each piece is coated in the caramel sauce. Use a rubber spatula to do this and not your hands! The caramel is VERY hot! (You can even divide the popcorn into two smaller cookie sheets, so you have more room to combine it with the caramel.)
Place the cake pan or cookies sheet in the oven, and bake for 45-50 minutes, stirring the popcorn every 10-15 minutes.
Remove from the oven when done, and allow the popcorn to cool. Once it is cool enough, break any large pieces and serve. Store in an airtight container or bag so the popcorn won't dry out.