The underground projects make sense. They afford added protection against attacks and destructive environmental hazards, like sand and severe weather. As Stratfor noted, Chinese personnel could use the space to conduct a variety of activities discreetly, too.
If nothing else, it adds significantly more storage room to what might otherwise be a cramped enclave. A portion of the underground part of the base likely includes hardened bunker areas for a command post and other sensitive operations spaces. With entrances that look large enough to drive vehicles in and out of, the PLA might be able to keep a portion of the base’s motor pool or other equipment safely stored away from the elements. It would be an ideal way of finding room for reserves of fuel, lubricants, and water, too.
The base needs these and other resources to both to perform its day to day functions, whatever they may be, and support PLAN task forces as they move in and out the area. If China hopes to make the base a regular port of call for larger groups of ships, including future carrier battle groups, it will need to have significant reserves of a number of items on hand.
It’s also worth noting that the largest area of underground construction at China’s Djibouti base sits under what otherwise appears to be a series of hangars and an attached aircraft apron that does not appear big enough at present to handle fixed wing aircraft, like fighter jets, surveillance planes, cargo aircraft, or even larger drones, but could support either helicopters or smaller drones. It is possible that fuel storage underneath could feed into pump stations or hydrants above ground.
Having all this on hand would give the base the ability to operate independently from local suppliers for at least a short period of time if necessary, too. Maybe part of the underground complex will contain a generating station to provide a separate and secure power supply, as well.
Whatever the purpose of this underground complex is,…