In the remote desert where China detonated its first atom bomb nearly 60 years ago, a drilling rig recently bored a deep vertical shaft that is estimated to plunge down at least a third of a mile. It is the strongest evidence yet that Beijing is weighing whether to test a new generation of nuclear arms that could increase the lethality of its rapidly expanding missile force.

For years, U.S. government reports and independent experts have expressed vague concerns about the old base, Lop Nur. The reports point to possible preparations for year-round operations and a “lack of transparency.”

Now, however, waves of satellite images reveal that the military base has newly drilled boreholes — ideal for bottling up firestorms of deadly radiation from large nuclear blasts — as well as hundreds of other upgrades and expansions.