Azovstal tunnels

"As per some estimates, Azovstal, owned by the oligarch Rinat Akhmetov has about 24 km of tunnels running at a depth of up to 30 meters.  Built back in the 1930s, the metallurgical plant with an extensive network of bunkers, an “underground city”, was supposed to become a refuge for people in case of war." 

Former Azovstal employee informed Russians of the tunnels' location

 

 source  frontierindia.com