Back in the 60s, the Cold War was in full swing, and the Russians, being Russian and therefore brave to the point of foolhardiness, created the Tsar Bomba, the "Father Bomb". In all fairness, they were actually being reserved when they built it; the final bomb was a 50 megaton monster, half the 100 megatons they wanted to make originally. I am going to guess that their scientists discovered something along the lines of "that much energy released at once will open a gateway to Gozer and its minions, which will eat our people, and more importantly, drink our Vodka."
So they reigned it in. The resulting bomb was still the most terrifying man made thing ever; even scarier than Bea Arthur nude, playing volley ball. The explosion was so huge, it nearly knocked the plane that dropped the bomb out of the sky. It was felt, seen, and even caused damage, 620 miles away from ground zero. Despite being an aerial burst (shown above), the explosion struck the ground and reached 8 km into the sky. The resulting fireball/mushroom cloud rose 40 miles into the sky.
The shock wave it generated circled the Earth 3 times, and was still measurable on the final pass.
Ironically, the idea behind making the Tsar Bomba was to create a "cleaner" nuke; you see, the larger and hotter the blast, the less radioactive fallout created by it. So the largest nuclear weapon ever detonated was actually the Russian attempt at making a "green" nuke, decades before the Green movement ever began.