Satellite photos, from sources like Google Earth, clearly show that China is building more underground storage facilities for its growing fleet of Su-27/30 warplanes. These aircraft are being build, under license, in China, as well as being imported from Russia. With over 200 of these aircraft, and more coming into service each year, China has an incentive to protect these high value targets. Taiwan is testing a precision cruise missile that could hit Su-27s parked in the open, or conventional above-ground concrete shelters. So taking it underground makes sense. These shelters are more expensive to build, and use (they require expensive ventilation, and more effort from ground crews to move the aircraft in and out.) But if China goes to war with anyone, these aircraft are their most modern, and only chance of maintaining control of Chinese air space.