Multilateral agreement signed by the US, CIS, UK, and 90 non-nuclear-weapon states would ban any and all nuclear tests, big or small, above and below the Earth's surface. It established a worldwide monitoring system - including 170 seismic stations - to check air, water and soil for signals that someone set off a nuclear explosion. 

India was the only known nuclear state that refused to sign the treaty. Pakistan also refused to sign unless India did.