From the Strait Times:

“It’s a city within a city,” said Mr Eduard Basurin, a representative for pro-Russian separatists in the eastern Donetsk region.

“There are several underground levels that date back to Soviet times which you can’t bombard from above. You have to go underground to clean them out, and that will take time.”

Entering the tunnels would be all but impossible for Russian troops, according to Mr Alexander Grinberg, analyst at the Jerusalem Institute for Strategy and Security.

“They can try, but they’ll be slaughtered because the defenders of the tunnel will absolutely have the tactical upper hand,” he told Agence France-Presse……

The Viet Cong made the vast Cu Chi tunnel system near Saigon a base for attacks against United States troops and Hamas has used tunnels in their fight against the Israeli army.

The Islamic State group’s tunnel system in Mosul, Iraq, allowed its fighters to surprise American troops with sudden appearances seemingly out of nowhere.

But the most memorable example dates back to World War II and the battle of Stalingrad with its fierce fighting in the Red October industrial complex……

 

The astute use of tunnels has lost none of its effectiveness in the 80 years since, rendering enemy artillery, air strikes and snipers virtually useless.

Satellite surveillance is also of limited use against combatants hiding below ground, as is technical intelligence, the spying on enemy weapons.

However, as analysts point out, the underground system works only if the network is vast and fighters have enough ammunition, food and water, which requires meticulous advance planning.

Soldiers deployed underground also need extremely good training to be operational in this unusual combat environment, said Mr James Rands at British defence intelligence specialists Janes.