On this day (August 6) in 1945 -- six years into World War II and four years after the bombing of Pearl Harbor -- the U.S. dropped a uranium bomb nicknamed "Little Boy" on Hiroshima, Japan. At least 80,000 people were instantly killed. Three days later, "Fat Boy" killed around 40,000 more people in Nagasaki, and in the weeks that followed, thousands more died from grotesque injuries that resulted from radiation poisoning. Hiroshima has been commemorating the anniversary in some heart-wrenching, beautiful ways.

While they didn't live through those events, young people in Hiroshima have been commemorating the 70-year anniversary in some heart-wrenching ways.

For a little background on why you're seeing photos of "die-ins" today, it's worth revisiting this period in history. The U.S. believed the only way to end World War II was to drop the atomic bomb. (The thinking was that any attempt to invade Japan through conventional means would result in too many American casualties.) Moreover, the devastating effects of radiation poisoning weren't fully understood prior to the fallout in Hiroshima.

Fierce debate over whether the use of the bombs was justified has continued in the 70 years since. The U.S. also remains the only nation to ever drop the bomb.

In a powerful act of remembrance, young people in Hiroshima performed a "die-in" in front of the Atomic Bomb Dome, the only structure left standing in the district after the bomb fell. At Hiroshima's Peace Memorial Park, representatives from around 100 countries and tens of thousands of visitors gathered for a ceremony of remembrance that included a minute of silence, ringing the "peace bell," releasing doves, and reading a declaration of peace calling for an end to nuclear weapons.

One of the representatives in attendance was U.S. Ambassador Caroline Kennedy, whose visit marks the third time we've sent an ambassador to Hiroshima for the anniversary of the bomb. On August 9, Kennedy will travel to Nagasaki to take part in that city's commemoration ceremony, too.

During the ceremony, Hiroshima's mayor, Kazumi Matsui, invited world leaders to visit Hiroshima and Nagasaki.

"President Obama and other policymakers," he said, "please come to the A-bombed cities, hear the hibakusha (surviving victims) with your own ears, and encounter the reality of the atomic bombings. Surely, you will be impelled to start discussing a legal framework, including a nuclear weapons convention."

As of 2014, 192,719 survivors of the Hiroshima and Nagasaki bombings were still alive in Japan. The Hiroshima Peace Memorial Museum has collected their stories and drawings, along with artifacts from the bombing, like the famous burned tricycle once owned by a 3-year-old boy named Shinichi, the subject of the children's book Shin's Tricycle.

People also took to social media to commemorate the anniversary of the bomb and say, #NeverAgain:

The city of Hiroshima is now fully dedicated to advocating for peace and to the end of nuclear weapons worldwide. Since 1968, each of the city's mayors has written a letter of protest to every country thought to be in possession of nuclear weapons, and the letters hang on the wall of the Peace Memorial Museum.