Comment by gilleain

Interesting, however that was 1966:

> the explosive was detonated at a depth of 1,500 m (1,640 yd) on 30 September 1966. The hole that the explosive was lowered into had also been sealed with cement so as to prevent atmospheric radioactive contamination.

From https://en.wikipedia.org/wiki/Nuclear_Explosions_for_the_Nat...

> The last nuclear explosion by the Program 7, codenamed Rubin-1, was performed in Arkhangelsk Oblast on September 6, 1988.

Although there is no separate page for that.