The theme of this paper is the siginificance of underground space in urban operations in the 21st century wars. In the introduction we go through the underlying definitions of the underground warfare and the use of tunnels, caves and underground spaces, either natural or man made. Through the historical overview of the use of the underground shelters for defensive purposes or tunnels made for attack, geological considerations for successful designation are also mentioned. In the second part of this paper we discuss the past of military conflicts, wars and military operations in which the underground space played an important role. In this section of the paper, through historical examples, we follow the development, desings and reasons for using underground facilities for attack or defense purposes. The paper states which counter-mesures were used to oppose opponents that used underground space in their tactics. Through this historical review, we return to ancient history and chronologically progress to the end of the 20th century, mentioning the most important conflicts in which the tactics of using the underground space was applied. In the third part of this paper, there is an overview of how and in what ways is the underground space used in 21st century conflicts, i.e. in urban operations carried out in the Middle East and Afganistan. Since there are conflicts between non state actors, primarily terrorist organizations, it is about asymmetric warfare because these organizations use the „guerrilla“ tactics to be able to fight against a technologically advanced opponent. The end of this paper covers new technologies and the ways of combatting in the underground structures, the ways to fight in them, how to detect and destroy them.