Fifty years ago this morning, Baneberry—a 10-kiloton, weapons-related, underground nuclear test 912 feet beneath the Nevada Test Site—accidentally vented, releasing 80,000 curies of radioactive iodine-131, more than any other US underground test. See: https://www.ctbto.org/specials/testing-times/18-december-1970-the-baneberry-incident/

NTS workers were quickly evacuated and 900 were radiologically surveyed. Of these 86 were decontaminated onsite, with 66 of these individuals sent to Mercury, Nevada, for thyroid measurements (chart). Eighteen of those workers received whole body count measurements in Las Vegas.

According to a 1971 AEC report, "no exposure on onsite personnel was in excess of the occupational standards in AEC or Federal Radiation Council occupational guides for normal peacetime operations." The highest exposures were received by two security guards in Area 12.

Those two security guards, Harley Roberts and William Nunamaker—who were ordered evacuate NTS workers in the path of the radioactive cloud and spent an hour inside it as a result—died of acute myeloid leukemia within four years of the accident. Their story is told in this book:

More than 500 privately-owned onsite vehicles were radiologically surveyed; more than 400 were contaminated. All but 86 "were decontaminated utilizing water spray and vacuum cleaners" and returned. The 86 more contaminated vehicles underwent a more thorough decontamination.

To detect and track offsite contamination, the AEC relied on a network of air sampling stations. In addition "the Standby Milk Network was activated in California, Idaho, Wyoming, Colorado, and Utah." Radioactive iodine-131 was detected in milk in all five states and in Nevada.

Further environmental sampling was conducted in those states and in Arizona, Minnesota, Montana, South Dakota, and North Dakota. The AEC estimated 4,000-5,000 people were in Tonopah, Nevada, and surrounding ranches and small communities where ground contamination was found.

Here is the Atomic Energy Commission's summary report about the Baneberry accidental venting, which continued for 24 hours after detonation. Because the cause was unknown, all underground testing was suspended for six months while the AEC investigated. https://www.osti.gov/biblio/4679984 

A 1989 report by Congress' Office of Technology Assessment, "The Containment of Underground Nuclear Explosions," had this to say about the cause of the accident: http://www.princeton.edu/~ota/disk1/1989/8909/8909.PDF