On 3 July 1974, in Moscow, the US and USSR sign the Treaty on the Limitation of Underground Nuclear Weapon Tests (Threshold Test Ban Treaty, TTBT) — the instrument capping underground nuclear explosion yield at 150 kilotons.

Signed by Nixon and Brezhnev, it complements the 1963 Partial Test Ban Treaty (banning atmospheric, outer-space and underwater tests) with a yield limit for underground shots.

Entry into force was delayed until 11 December 1990, after the 1990 verification protocols. It remained a pillar of arms control until CTBT efforts.

The TTBT reduced high-yield tests and opened the way for bilateral seismic verification.

For arms control, 3 July 1974 marks the capping of underground nuclear tests at 150 kt.

The official text is on the State Department site / treaty collections.

The Threshold Test Ban Treaty of 3 July 1974 remains the milestone capping US–USSR underground nuclear tests.

Image: Moscow / nuclear tests; Wikimedia Commons. Cropped to 16:9.

Source consulted: Threshold Test Ban Treaty. Treaty text: Threshold Test Ban Treaty (US State Department).