Grease a large cookie sheet. Set aside.
In a heavy 2 quart saucepan, over medium heat, bring the sugar, corn syrup, salt, and water to a boil. Stir until sugar is dissolved. Stir in peanuts. Set candy thermometer in place, and continue cooking. Stir frequently until temperature reaches 300º F (150º C), or until a small amount of mixture dropped into very cold water separates into hard and brittle threads.
Remove from heat; immediately stir in butter or margarine and baking soda; pour at once onto cookie sheet. With 2 forks, lift and pull peanut mixture into rectangle about 14×12 inches; cool. Snap candy into pieces.