The location is well known because that bunker was built under the USSR. And it is strong/deep enough that only a powerful nuclear explosion would endanger it.

The location is well known because that bunker was built under the USSR. And it is strong/deep enough that only a powerful nuclear explosion would endanger it. This is a very serious bunker we're talking about. What they [Russians] might not know is all the modern exits and connections (to the metro system and elsewhere) that have been built since then, or things like supply chains for food, location and current state of the air filtration or communication systems. BTW, since we're talking about bunkers: the US military in their what-if-shit-hits-the-fan plans has a very hard nut to crack with the Chinese air force, which brilliantly built their aircraft hangars deep inside mountains. It may not be immediately obvious, but the best way to counter an air force is to destroy the aircraft before they take off. Doesn't work if the hangar-bunker is 100% resilient to any kind of a conventional bomb that you could drop on it.