Transcript for Nearly 2 years after inferno: A look at Notre Dame's restoration progress
Finally tonight here, it's been nearly two years since we first took you inside notre dame, after that historic fire. Tonight, our James Longman has gone back. Reporter: It was nearly two years ago, the world watched notre dame burn. The spire collapsing. Tonight, we were invited back in. 200 tons of lead melted into notre dame, poisoning they are now salvaging what they can and cleaning it. We're actually not even allowed into this part. Because they're so worried about lead inhalation. You need a specialized mask in order to go in. You can see the worker there with his mask. Remember, just one month after the fire, they showed David the cross? It was still standing. General, this is cross that the world saw. Yes. The entire world saw the cross. Reporter: It has now been removed for safety. And remember the charred stones? The stones are coming from the vault here. These are the stones that came from the roof? Yes. Reporter: Well, now archaeologists face the painstaking team of piecing it all back together. They keep all these blocks in here. And at every stage of the refurbishment, they come in to get what they need. And she says they've had some unique help from an American. So, an American architect did a plan of notre dame and that's the one you're using? Oh wow. Back in 2010, U.S. Art historian Andrew Tallon meticulously drew this part of the church. And his drawings are now helping to rebuild. And remember the holes in the roof they showed David? And the robot on top of the debris because of the danger. These are robots because people cannot go here. We cannot expose the life of the worker. Reporter: And take a look at it now. This is the work under way. The most important work at the moment is the work to stabilize this building. Tonight, we go to the roof to get a better view. Heading up to the roof. Which of course is where all this began. Workers rebuilding. The eiffel tower in the there are now anywhere between 150 to 200 people working on notre dame at any one time. Slowed down, of course, by the pandemic. Take a look at that charred wood. That's what remains of the oak vault that held the ceiling up. You can see there the sailors behind me retrieving bits of debris from the roof, which is then sailed all the way down the side and taken to the archeologists for examination. Those vital remains help the team understand notre dame and reild it as the world watches and waits. Wow. Our thanks to James and the team for taking us back. The slow and steady work to bring it back. Good night.
This transcript has been automatically generated and may not be 100% accurate.