What is the nuclear test ban treaty and why is Russia changing its position?

The CTBT is "a remarkably effective treaty, even though it hasn't come into force", said Matthew Harries, director of proliferation and nuclear policy at the RUSI think-tank in London. The treaty's verification regime includes an International Monitoring System (IMS) - a network of observation facilities around the world that are capable of detecting the sound of a nuclear explosion or the associated seismic activity or radioactive fallout.