A nuclear weapon doesn’t need to be used in war to have lasting impact. More than 2,000 such weapons were tested during the 20th century, leaving behind generational fallout that affects human beings, public health and the environment. The American government has not finished cleaning up the consequences of testing that ended decades ago, and the possibility of restarting today is real.

As W.J. Hennigan details in his latest for Times Opinion’s “At the Brink” series, the United States, Russia and China are all modernizing their testing facilities. None of the nations have conducted an underground nuclear test since they all signed the 1996 Comprehensive Nuclear Test Ban Treaty. But the United States and China never ratified the document into force, and Russia rescinded its ratification in November, a step backward for international arms control.

Now, commercial satellite imagery provided by Planet Labs PBC shows that all three pre-eminent nuclear powers are modernizing and expanding their testing infrastructure, adding new buildings, cutting new roads and boring new tunnels in recent years. The photos, analyzed by the James Martin Center for Nonproliferation Studies at the Middlebury Institute of International Studies, are not proof a test is imminent, but they do indicate preparations are being made if political leaders ever decide to give the go-ahead.

The United States says it’s being transparent about its expansion in Nevada, where it previously conducted 928 nuclear tests. It’s building a state-of-the-art underground lab that conducts subcritical tests, or experiments that use explosives on components of a weapon but fall short of triggering a nuclear chain reaction. The surrounding support facilities, the National Nuclear Security Administration says, are for “security, food, housing, and administrative needs” for on-site personnel.