How did China prepare for West Asia war by building underground caverns?

China built a surplus of 1.24 million barrels of oil/day this year ahead of the Iran war, helping it stay insulated from oil supply crisis and price shocks. With a combination of underground caverns and above-ground tanks, China has a storage capacity of 1.1-1.2 billion barrels. This helps China buy when prices are low and use reserves when prices jump.