Preheat oven to 170 degrees F.
Cover a large cookie sheet with parchment paper. Set cookie sheet in oven to keep warm.
Into a large saucepan over medium heat, add sugar, corn syrup, salt, and water. Stir continuously for 3 minutes, or until sugar is dissolved.
Add peanuts and continue stirring until temperature on a candy thermometer reaches 300 degrees F.
Remove from heat and quickly stir in butter and baking soda then immediately pour entire mixture onto cookie sheet. Use a buttered spatula to help spread the mixture into an even layer.
Allow to cool completely then gently lift peanut brittle from the cookie sheet and use your hands to snap the candy into pieces.