Here's what the 'Little Boy' atomic bomb dropped on Hiroshima would do to major American cities

Seventy years ago, the US Air Force's "Enola Gay" B-29 Superfortress bomber dropped a 15-kiloton nuclear bomb, code-named Little Boy, on the Japanese city of Hiroshima.

The blast, which was the first detonation of a uranium-based nuclear device in history, instantly killed 70,000 people, while the lasting effects of the radiation brought the toll up to 140,000.

Hiroshima, a city of 310,000, was almost completely annihilated.

The nuclear chain reaction unleashed by a mere two pounds of concentrated uranium atoms, created when two hemispheres containing a total of 140 pounds of high-enriched uranium slammed into one another about 1,900-feet above Hiroshima, created an over 1,000-foot fireball, ended tens of thousands of lives, and vaporized an entire city.

It's impossible to truly grasp the enormity of an atomic blast like the one that leveled Hiroshima, but The Nuke Map is an invaluable attempt at it.

The work of Alex Wellerstein, a historian of nuclear technology at the Stevens Institute of Technology, the Nuke Map lets users detonate bombs of various yield over any point on earth and then calculates potential casualties, fatalities, and fallout.

The tool gives a jarring perspective on the Hiroshima bomb by allowing users to superimpose Little Boy's blast radius over a variety of familiar locations. In Washington, DC, a 15-kiloton bomb, with an explosive yield equal to 15,000 tons of TNT, would kill 126,000 people and injure over 190,000 more:

A 15-kiloton nuclear weapon has a fireball radius of over 500 feet, giving the most destructive section of the explosion a width of over four Manhattan blocks.

If detonated over 20th St and 5th Avenue in Manhattan, the air-blast radius of Little Boy-sized device - a zone where increased air pressure would crush most buildings and where the casualty rate would be in the 100% neighborhood - would span from the the East Village to the southern edge of Midtown. The bomb would kill an estimated 445,000 people.

Another look:

The bomb's enormity can also be glimpsed by dropping it on mid-sized cities. Little Boy would irradiate the entirety of downtown Grand Rapids, Michigan, and kill 32,000 people out of a population of around 500,000.

The examples of Hiroshima and the attack on Nagasaki 3 days later convinced the world of the bomb's destructive potential and created a still-vital sense of urgency among diplomats, politicians, military planner, and activists for ensuring that atomic devices are never used in war.

But there are still nearly 16,000 nuclear weapons on earth, many of which have a far higher explosive yield than Little Boy. It's been 70 years since the last nuclear strike. But if that streak ever comes to an end, the cost would be unimaginable.

Hiroshima ended up being the second-to-last offensive nuclear strike in history - but only so far.