In a large, microwave-safe bowl, mix sugar, butter, syrup and salt together.
Microwave on high until mixture is boiling, then continue to boil for 2 more minutes on high.
Mix baking soda into mixture.
In a large (clean) brown paper bag, pour mixture over popcorn and shake vigorously.
Microwave in bag for another 30 seconds.
Take out of microwave, and shake again.
Repeat process at least two more times to ensure even coverage of caramel on popcorn.
Immediately pour out on a baking pan to cool.
For a lighter, more buttery taste, substitute white sugar for the brown.
For extra crunchiness, mix 2 cups peanuts, almonds, or other nut of your choice with the popcorn prior to mixing the caramel.
For chocolate lovers, microwave for 90 seconds, stirring once, 1 cup chocolate chips and 1 tsp shortening and drizzle over caramel corn.
For extra spiciness, add 3/4 tsp ground cinnamon, 1 1/2 tsp freshly grated nutmeg, 1/8 tsp ground cloves, and 1/4 tsp ground cardamom to the caramel mixture.
For easy transport (especially for Halloween treats): after removing from microwave for the last time, grease hands and form large handfuls of the carmael corn mixture into balls, then set aside to let cool.