The activity has raised concerns that Iran may be turning the facility into a uranium enrichment site.
The site is buried deep beneath solid granite, and the US currently has no known bunker-buster weapon capable of reaching it.
Granite is extremely hard, making it a major barrier for conventional bunker-busting weapons.
Pentagon has created a highly secretive task force to speed up development of a new weapon that could potentially reach the facility.
US also awarded an emergency $1.2 million contract for weapons testing at a classified underground test range in New Mexico, linked to simulating strikes on Iran’s deepest bunkers.
Even a tactical nuclear weapon would not guarantee the complete destruction of a facility buried this deep and built to withstand powerful attacks.