In a large saucepan, combine the sugar, water, butter and corn syrup and bring to a boil.

Cook over moderately high heat, stirring constantly, until the caramel is light brown and registers 300° on a candy thermometer, about 10 minutes.

Remove from the heat and carefully stir in the baking soda. The mixture will bubble. Stir in the nuts, then immediately scrape the brittle onto a large rimmed, nonstick baking sheet. Using the back of a large spoon (oil it lightly if it sticks), spread the brittle into a thin, even layer. Sprinkle with salt. Let cool completely, about 30 minutes. Break the brittle into large shards.