U.S. Secretary of Energy Ernest Moniz, U.S. Secretary of State John Kerry and U.S. Under Secretary for Political Affairs Wendy Sherman (L-3rd L) meet with Iranian Foreign Minister Mohammad Javad Zarif (2nd R) at a hotel in Vienna, Austria June 28, 2015.
It's crunch time in Vienna.
Iran and the P5+1 group (the five permanent UN Security Council members, US, France, China, Russia and the UK, plus Germany) officially have until Tuesday to come up with a final deal on Iran's nuclear program.
It has taken months of planning and preparation to get the sides to the table in order to negotiate a deal. Both Iran and the US have a lot to gain if diplomacy wins and lose if it fails.
But what if they fail?
"If there is no deal, there are more attempts at more diplomacy and more economic sanctions," says Michael Crowley, senior foreign affairs correspondent for Politico.
Of course there is also a scenario where those talks and sanctions fail, or, where Iran decides to make a nuclear bomb.
Crowley says, for a while now, the US has been preparing for such a case, as he writes about for Politico.
One idea is that if Iran goes on a path to create a nuclear bomb, then the US would consider bombing its nuclear facilities. But that, is no easy task.
"The challenge, if you need to resort to a military option in Iran, is that Iran's uranium enrichment facilities are under ground," Crowley says.
One of those facilities, called Fordow, is located deep in a mountain in an area near the city of Qom, two and a half hours from the capital, Tehran. To destroy Fordow, the US needs a very powerful and smart bomb: The Massive Ordnance Penetrator or MOP. It's the largest, non-nuclear bomb in the US arsenal.
"I liken it to a 20-foot, 30,000-pound nail being driven into the rock, which then detonates with massive force," explains Crowley.
According to Crowley, in recent years, the US increased its work on the MOP project.
"As recently as a few years ago, there was a belief that we would not be able to hit Fordow or other similar facilities that we may not know about with our existing bunker-busting arsenal," Crowley says. "Development was fast tracked and there were modifications made as recently as this past winter."
Crowley says the Pentagon has been open about the MOP, which it views as a major deterrent.
"[The Pentagon] does not mind Iran knowing that we have this option, because the feeling is we need this leverage," he says.
Meanwhile, if it does come to a point where the US decides to use military action, and it needs to act fast, getting other members of the P5+1 on board might turn out to be a challenge.
Russia and China would likely oppose an attack on Iran, while it might be difficult to find the political will in the remaining countries.
For now, Crowley hopes things don't have to come to that.