Explanation

• 'So' is placed before a modifier — adjective or adverb — to intensify its quality or manner.
• ‘So’ emphasis on the modifier expresses a remarkable situation.
• This clause can stand alone as a sentence, or it can be paired with 'that' + clause to express an effect (result) of the remarkable situation mentioned in the main clause.
• The United States is so big that there are five time zones.