Tsar bomb. Russia recently declassified some of its footage. Videos have details on where it was set off, it's size and cloud dimension and height.
When we first came to know about this world shaking 60 mega ton explosion, these details weren't available, just assumptions from US side.
Now the footage are here.
This was the largest ever nuclear explosion by a big margin. Everything Russian during the cold era were large. The largest bomb, largest aircraft, largest submarine. But not necessarily economical. Had the pride value, but not the winning edge. Cascade of layer and layer of this -- and ultimately the Soviet Union failed into a pile of rubbish that had lost it's core's strength far earlier.
The cold war.