Key sites are spread across the country, but most attention is focused on to the tents on fordo.

Iran initially hid the existence of both sites, fueling concerns that it might be secretly trying to build a nuclear bomb.

Now both sites to the subject of intense scrutiny.

Natanz was first exposed in 2000, and two.

Thes satellite images reveal big changes of the site.

You can see better roads and extensive new buildings by 2020 while fordo, a remote site built deep in a mountain, was exposed in 2000 and nine by Western intelligence.

Again, look at how the buildings and roads developers the years passed.

What makes thes sites important is that they aware the enrichment of uranium is taking place.

So what exactly is enrichment?

Well, it's the technically challenging process in which uranium gas is fed into machines called centrifuges, and this is what they look like.

Slender silver cylinders here being shown to Iran's president.

Centrifuges spin the uranium at incredibly high speeds.

This separates out the lighter uranium to 35 atoms from heavier you 238 atoms for low enriched uranium is where you have 3 to 5% of U 235 This could be used as fuel in a nuclear power plant if you keep enriching it so there's a higher percentage.

It can be used in research and medical applications.

On def you enrich.

It'll the way up to 90% then it could be used to make a nuclear bomb.

Iran insists its nuclear program is purely for peaceful purposes, but others fear it is seeking a weapon.

Today could have been the end of hope on this issue, but now we are starting.

A new chapter of Hope on International Deal was signed in 2015.

It limited Iran's stockpile of uranium on said it could only be enriched to low levels less than 5%.

It limited to 5000 the number of centrifuges and required inspections to ensure Iran complied.

The restrictions were set to last for up to 15 years.

In return, Iran was promised that sanctions would be lifted, but not everyone was happy.

The Iran deal is a disaster.

I think it's terrible.

One of the worst and most one sided transaction in 2018, then President Trump pulled the US out of the deal, saying it was flawed.

And after that, Iran increased its enrichment activity.

These once secret sites have bean at the heart of a long running crisis on what takes place here will determine when and if it's resolved.