 The battle was also noted for the pioneering use of tunnel warfare by the Chinese in the Korean War . During the fighting , a company of Chinese soldiers had defended their positions from a U @-@ shaped tunnel capable of housing 100 men , which had served as both a bomb shelter and a base for counterattacks . The company leader later claimed that the tunnel enabled the defenders to inflict 700 UN casualties while suffering only 21 casualties in return . Impressed by the report , the commander of the People 's Volunteer Army , <unk> <unk> , later ordered the construction of 30 @-@ metre ( 98 ft ) deep tunnels along the entire front line , and it formed a formidable obstacle for UN forces to overcome during the stalemate period . 
