Read the following to get info about the pictures.
Abstract
The PLA's network of around forty underground airbases provides a unique and indeed superlative capability to withstand a large scale counter-air campaign, as significant effort with penetrating munitions of large mass would be required to close such bases down for the duration of the campaign, or permanently render them unusable.
As these bases deny surveillance by orbital, airborne or ground based ISR assets, they provide an inherent capability to perform covert preparations for combat operations, be it in times of conflict, or in the period immediately preceding a conflict.
The hangaring capacity of these underground bases is of the order of at least 1,500 combat aircraft, possibly greater, if more than basic internal tunnel arrangements have been employed. This capacity represents more than the total number of modern combat aircraft currently operated by the PLA.
In strategic terms, the PLA's underground airbase infrastructure provides a capability to deny at this time even the United States the opportunity to inflict massive early attrition upon the PLA's fleet of combat aircraft, while these are on the ground. This would force any opponent, including the United States, into a protracted aerial war of attrition, before decisive losses could be inflicted upon the PLA combat aircraft fleet.
The inherent survivability of the PLA's impressive underground airbase infrastructure has not been a major consideration in the ongoing debate in the United States on the utility, and indeed intended design, of the replacement heavy bomber aircraft. If the defeat of the PLA's underground airbase infrastructure is part of the intended role of this future aircraft, then this aircraft will need to combine the survivability needed to repeatedly penetrate what is becoming the most capable Integrated Air Defence System in existence, with the ability to deliver heavy earthquake bombs. Anything less will result in unsustainable combat attrition.
Actual source (which has not been disclosed by OP):
Assessing PLA Underground Air Basing Capability
Check it out, its interesting; and another good job by
Sean O'Connor, who Lankan Ranger does not like to acknowledge.