Transcript for Workers evacuated at nuclear waste site
That's the problem. And now that main scare at the largest nuclear waste complex in the country, workers at the Hanford nuclear reservation in Washington state were evacuated after part of a tunnel collapsed. The site houses a large portion of some of the most radioactive material in the world and ABC's Matt Gutman is on the scene with more and good morning to you, Matt. Reporter: Hey, good morning, Michael. There are huge crews monitoring the site 24/7. Now, at first we were told it was just a dip in the ground then that it was the size of a kiddie pool, hours after that it grew to the size of a large swimming pool. Now overnight we were told that the hole itself does have radiation but so far it's not spreading. Respond to the purex tunnel for a possible collapse of the tunnel. Reporter: Hundreds of workers at the Hanford nuclear site 200 miles from Seattle were forced to take cover after a portion of a tunnel collapsed where radioactive materials are stored. They told us to take cover to make sure we were safe and accounted for. Reporter: In these aerials you can see right into the tunnel holing that radioactive material now a gaping hole triggering fears of a radioactive leak. The cave-in taking place where two tunnels link to a facility called purex, plutonium was extracted for nuclear weapons here until 19780. Hanford has been closed for 30 years, the only work there, the cleanup of enough radioactive debris to fill the Superdome. It's going to take a significant federal workforce and state oversight to clean up the enormous mess left by the weapons production process. Reporter: The energy department says while the cleanup there is ongoing, there was no indication of a release of contaminant at this point. Crews are still testing the area. There was contamination at that site, yes. It didn't spread beyond that. Reporter: Disturbingly the cause of the cave-in remains a mystery. Too early to know what caused the roof to cave in, may not know that for some time. Reporter: Now we're told that those tunnels were constructed in the '50s out of wood and concrete. There is a threat, however officials say if there is a storm or windstorm of some sort it could blow that radiation miles around and that could threaten the public. Also there are 42 miles of trenches stuffed with radioactive material all around so officials don't want to do anything rash to try to ameliorate the problem before they know what happened here. Guys. Very sensitive situation. That's right.
This transcript has been automatically generated and may not be 100% accurate.