Beginning in 1949 and spanning a period of 40 years, the Soviet Union carried out more than 450 nuclear tests in the isolated steppes of eastern Kazakhstan. In 1989, when the socialist state collapsed, the Russians pulled out and left the Kazakhs to their own devices—literally. Enough fissile material for a dozen or more nuclear weapons was left behind in mountain tunnels and bore holes, virtually unguarded and vulnerable to scavengers, rogue states, or potential terrorists.

In a remarkable and highly secretive feat of collaboration among the United States, Russia, and Kazakhstan, engineers and nuclear scientists from the three countries spent 15 years and $150 million to secure many of the tunnels and test areas at the sprawling Semipalatinsk Test Site. Siegfried S. Hecker, a senior fellow at Stanford University’s Center for International Security and Cooperation, launched the project while director of the Los Alamos National Laboratory. He used his personal ties with Russian scientists to prod them into working with the Americans and Kazakhs after a visit to the test site in 1998 left him stunned by the lack of security and the presence of scavengers.

It was one of the greatest nuclear nonproliferation stories never told, until the White House and Pentagon revealed some details in 2012, which David Hoffman and Eben Harrel of Harvard’s Belfer Center made public over the weekend in an in-depth report, Plutonium Mountain. In October 2012, officials from Kazakhstan, Russia, and the United States dedicated a monument that simply reads: The world has become safer.

In this interview, Hecker—who teaches the popular Stanford class Technology and National Security with former Secretary of Defense William J. Perry—speaks with the Bulletin about the extraordinary Semipalatinsk mission and talks about next steps to secure the site.

More: Into Thin Air: The Story of Plutonium Mountain