Nuclear tests

War History Online has an article about the ten biggest atomic explosions:

Most of us have seen footage or photographs of the explosions at Nagasaki and Hiroshima: the mushroom clouds, the radius of devastation. Those, however, were not even close to the biggest nuclear bombs ever detonated.
The bomb dropped on Nagasaki, dubbed Little Boy, was four kilotons, and Fat Man dropped over Hiroshima, was about twenty kilotons. Just a few years later, the US was testing nuclear bombs in the megaton range, and the then-USSR followed suit in the early 1960s.
As horrifying as the bombs of World War Two were, they are nothing compared to those of later years. Thankfully, the Americans and the Russians have more recently begun to test non-nuclear bombs. Hopefully, no one will ever use them.

10. Trinity: 

The first atomic bomb test was the Trinity explosion, performed in New Mexico on 16 July 1945. At twenty kilotons, it wasn’t much larger than Little Boy and nearly the same size as Fat Man. Robert Oppenheimer, technical director of the Manhattan Project, said of this event: “If the radiance of a thousand suns were to burst at once into the sky, that would be like the splendor of the mighty one…" Then he quoted the Bhagavad Gita: Now I am become Death, the destroyer of worlds. 

9. Soviet tests 158 and 168 

The Soviets were delayed in the pursuit of nuclear arms by the ground battles off World War Two but, when the war ended, they jumped right into the race.
By 1949, they were ready to test First Lightning (photo), becoming the second nation to detonate a nuclear device.
Thirteen years later, they caught up to the US yet again with two atomic bombs, 158 and 168, that yielded ten megatons each. Not surprisingly, there are no photographs available of these tests, done in the Novaya Zemlya area of Russia. 

8. Ivy Mike

The very first hydrogen bomb, Ivy Mike, completely obliterated its testing site. Elugelab Island in the Marshall Islands was deliberately built up for the 1952 test, but all that was left after detonation was a crater nearly two hundred feet deep.
The ten-megaton bomb had a mushroom cloud extending thirty miles into the sky, with a crater radius over one mile.
The size and speed of the blast were awesome, in the pre-1980s sense of the word. The three-mile wide fireball mushroomed over ten miles in the first ninety seconds, until reaching its zenith of a twenty-mile wide stem and hundred-mile wide crown.
The fallout and reaction of the ocean water to the circular shockwave were tremendous. Ships thirty miles distant were pelted with irradiated coral, and the vegetation on surrounding islands was wiped out.

7. Castle Romeo

Castke Romeo was part of a series of tests performed on Bikini Atoll in 1954. It had the distinction of being the first device tested over open water, rather than on a reef.
Detonated on a barge over the exact spot of the crater formed during the Castle Bravo test, the eleven megaton bomb is the third largest ever tested by the United States. Had it been detonated on an island or on land, it would have destroyed everything within two square miles. The most often printed photograph of a nuclear explosion is one taken of Castle Romeo. 

6. Soviet test 123

Test 123 preceded 158 and 168 by one year at the same location over Novaya Zemlya. No one knows why they decreased tonnage, but this prior test was just twelve megatons; just two over the tests of the following year. As with the others, no photographs exist. 

5. Castle Yankee

Castle Yankee exceeded expectations. Great for a job, scary for a bomb. Another in the series of the Bikini Atoll tests of 1954, the testers intended Yankee to give a yield of six to ten megatons, but it went to more than thirteen. Yankee was thirty-five miles high by a hundred miles wide. Its expanded horizons were due in part to fast fission of uranium tamper. Owing to wind conditions (65 knots at forty thousand feet), the fallout carried seven thousand miles and fell on Mexico City.

4. Castle Bravo

Despite the impact that movies and nuclear fairy tales have on our collective imagination, the largest nuclear blast credited to the United States was detonated over sixty years ago, on 28 February 1954:

Another in the series of Castle tests, Bravo also went above and beyond its projected six megatons, to fifteen megatons. The devastation had far more reach and destruction than Yankee’s later debacle affecting Mexico City.
The dry fuel thermonuclear hydrogen bomb was visible from over two hundred miles away in the first second. The first ten seconds saw a mushroom cloud five miles across and, within a minute, it had ballooned to nine miles high and seven miles across, and peaked ten minutes later at twenty-four miles high and sixty-two miles wide. The resulting crater was nearly a mile and a half across by two hundred and fifty feet deep.
The distance of the effects was enormous in scope. Close in, the fallout rained down on residents of the Marshall Islands, an island twenty miles distant was the scene of a fire ignited by the initial thermal flash, and sailors on a Japanese fishing boat eighty miles away were poisoned by fallout.
It has long been common knowledge that nuclear testing is horrifically harmful, but it was this incident that first created a demand for a ban on thermonuclear testing. The radiation from the blast reached over seven thousand square miles and caused terrible health to those exposed and birth defects in their children. 

3. Soviet tests 173, 174, and 147
Despite what had gone on in the Pacific in 1954, the Soviets continued to test. In the fall of 1962, they tested the third, fourth, and fifth most intense nuclear blasts at around twenty megatons each. Again, no photos are forthcoming. 

2. Soviet test 219
They amped it up in December of that year, with test 219. (Make a note to never travel to Novaya Zemlya.)
Test 219 yielded over twenty-four megatons. If anyone had been within two thousand square miles at the time (and maybe there were) they would have suffered 3rd-degree burns. Again, no photos.

1. The Tsar Bomba

A year prior to 219, the largest nuclear weapon ever tested was detonated at the Soviet's favorite test site, but with glass shattering effects over five hundred miles away.

The Tsar Bomba was the largest man-made explosion ever. The massive yield was between fifty and sixty megatons, and its flash was seen over six hundred miles away.

Detonated on 30 October 1961, its third-degree burn radius would have been four thousand square miles. The bomber that dropped it nearly got destroyed when the massive fireball simultaneously touched the ground and shot to six miles in the air. Putin might say that Mother Russia makes no mistakes, but Tsar Bomba, aka AN602, was only meant to go as high as two and a half miles. They had scaled back from their original plans for a hundred-megaton terror, but decided in the end that the fallout would be too great.

Scandinavians are surely thankful, as the effects on them would have been greater than the blast they saw before their windows broke out. Norway and Finland suffered damage to buildings, and sadly, a village thirty-four miles from the test site was incinerated.

While the US and Russia appear to no longer be testing nuclear weapons, they are testing bombs and explosive devices that are as devastating to the immediate bomb site. Lucky for the world that fallout will not be as large a problem with these alternatives. The most horrific thing about these bombs, called thermobaric bombs, is that those on the fringe that don’t die from the blast, but die from a vacuum-like effect on their organs which, essentially, contract and implode.

Rico says we've been really lucky that only Hiroshima and Nagasaki were under one of these...