Concrete.

This is coolbert:

As it was for the ancient Romans so as it is today too for the Iranian?

American [and Israeli too for that matter] bunker-busting bombs beware?

"Smart concrete".

"Iran makes some of the world’s toughest concrete. It can cope with earthquakes and, perhaps, bunker-busting bombs"

"A DUAL-USE technology is one that has both civilian and military applications. Enriching uranium is a good example. A country may legitimately do so to fuel power stations. Or it may do so illegitimately to arm undeclared nuclear weapons. Few, however, would think of concrete as a dual-use technology. But it can be. And one country—as it happens, one that is very interested in enriching uranium—is also good at making what is known as 'ultra-high performance concrete' (UHPC)."

“... unlike conventional concrete, Iranian concrete is mixed with quartz powder and special fibers – transforming it into high performance concrete that can withstand higher pressure with increased rigidity. What this all translates to is excellent building material given the environment that has found peaceful applications like the construction of safer bridges, dams, tunnels, increasing the strength of sewage pipes, and even absorbing pollution.”

ALL MANNER OF PEACEFUL APPLICATIONS. AND APPLICATIONS OF A MILITARY NATURE TOO WE MUST ASSUME. BUNKERS THAT WITHSTAND EVEN A PENETRATION BOMB DIRECT-HIT?

Able to even defy ordnance of the World War Two Grand Slam or Tall Boy variety?

See additionally from the History.com Internet web site the Roman use of concrete. Better than modern  concrete.

coolbert.