North Korea is not the only nation in East Asia to have built underground airbases. China has the largest number of these in the world (about 40). These facilities have the capacity to protect at least 1,500 aircraft. The airstrips (for takeoffs and landings) are not underground, but the storage and maintenance areas are, along with fuel, ammo, and maintenance and airstrip repair equipment.
These bases make it difficult for anyone to destroy Chinese aircraft on the ground. While the airstrips can be heavily damaged from the air, the repair equipment and airfield engineers are under cover and can quickly repair the worst damage. The U.S. Air Force has developed a number of ground penetrating bombs for facilities like this. Although these “bunker buster” bombs are usually mentioned in connection with Iran, its North Korea and China that have far more underground military facilities. The U.S. continues to develop these ground penetrating bombs and are always looking for new details on the construction of underground Chinese facilities.
These bases provide considerable protection from surprise attacks and give China an edge in any protracted war. The U.S. would be using bases in the region that are not underground and sometimes do not even have concrete hangers for all aircraft. China makes heavy use of ballistic missiles (most with a range of 300-700 kilometers and carrying a half ton warhead). There are nearly 2,000 of these and many are aimed at airfields in neighboring countries. The missiles are mobile and also have underground shelters. This extensive use of underground shelters greatly increases the capabilities of Chinese airpower.