It began with a blinding flash and a deafening boom. Then the shockwave arrived, hurling the young boys into the air and sending shards of glass from exploding windows into their skin.

Only later, as they made their way through the hellscape where their thriving city had once stood, did the boys realize that they were the lucky ones.

"There were fires burning everywhere, the city was a firestorm. The blue sky turned gray, and the night was black. We looked for mom, crying as the black rain soaked us," Okihiro Terao recalls.

That's when the "ghosts" appeared. Human-like shapes with undefined features emerging from the darkness, writhing and moaning in pain as they reached out to the living. The strange figures couldn't possibly be people, Terao remembers his 4-year-old self thinking.

"Their appearance -- it was hard to see who they were -- they were unrecognizable. I think that's why I was so scared," says Terao, now 82.

These nightmarish memories are of Hiroshima, Japan, on August 6, 1945. The young Terao had just survived the world's first nuclear attack.

At 8:15 a.m. Japanese local time that morning, the Enola Gay, a US Army Air Force B-29 Superfortress, had dropped a single bomb over the city and its approximately 350,000 residents.

That bomb detonated 580 meters (1,870 feet) above Hiroshima, killing tens of thousands of people instantly; some vaporized in temperatures reaching 3,000 to 4,000 degrees Celsius.

That was just the beginning. Hundreds of thousands would die in the days, weeks, months and years that followed; victims burned beyond recognition -- the "ghosts" of Terao's memory -- and those who died slowly from injuries related to radiation, a new phenomena the world was yet to understand.

Today, almost 80 years later, as world leaders descend on Hiroshima for this weekend's Group of Seven summit, Terao's memories have all come flooding back.

Doomsday redux

High on the agenda for the leaders of the world's biggest democracies as they meet in this highly symbolic city is Russia's invasion of Ukraine, with the Ukrainian President Volodymyr Zelensky expected to attend the summit in person.

According to the Bulletin of Atomic Scientists watchdog, Moscow's unprovoked invasion of its neighbor has taken the world closer to nuclear catastrophe than at any time since 1945.

Russian President Vladimir Putin, who is in charge of the world's biggest nuclear arsenal (with 4,477 nukes compared to the US's 3,708, according to the Stockholm International Peace Research Institute), has repeatedly dialed up his rhetoric about his willingness to use his nukes.

And with his unprovoked invasion not going his way, some fear what a cornered Putin might resort to.

"Russia's thinly veiled threats to use nuclear weapons remind the world that escalation of the conflict -- by accident, intention, or miscalculation -- is a terrible risk. The possibility that the conflict could spin out of anyone's control remains high," the Bulletin of Atomic Scientists said in January when it updated its Doomsday Clock, a measure of how close it thinks the world is to that nuclear disaster.

For Terao, the idea the world is hurtling back toward the nightmare he barely survived is incomprehensible.

"I think it's crazy that Russia is threatening to use nuclear weapons -- just verbalizing the thought makes me sweat, and as I say those words, blood rushes to my head," he tells CNN.

As he recounts his experience of the morning of August 6, 1945, it is not hard to see why.

Day the sky turned black

Back then, Terao had been living with his mother and two brothers in a rented second-story room about four kilometers (2.5 miles) north of the city.

He and one of his brothers had been playing outside when they saw the blinding light and turned and ran for the door of their home.

It was not until they reached it, moments later, that the shockwave from the blast lifted them off their feet.

Glass from shattered windows peppered their bodies. "We cried so much," Terao recalls.

But they were the "lucky ones" -- among the very few whose home hadn't collapsed.

They rushed upstairs, where they found their aunt clutching their younger brother, but they could not find their mother. She had headed out that morning to collect some belongings from their previous residence, just 300 meters from what is now known as the Gembaku or A-bomb dome, famous for being the only building in the immediate area to have survived the blast.

Together with their aunt, the boys headed into ground zero to find her.

As they walked, survivors covered in burns streamed in the opposite direction. Fires burned all around and black rain began to fall.

Miraculously, the boys heard the familiar voice of their mother Shizuko calling out.

Worried about the things she'd left behind in their former residence, Terao's mother had set out on the day of the atomic bombing to collect a few more things. She had been 1,000 meters from their home when the bomb detonated.

"It sounded like my mom, but we didn't know where she was. Then the voice started feeling closer -- that's when all the emotion I'd been bottling up burst out, and I started sobbing," he says.

"It seemed my mom recognized my aunt's figure ... she found us, especially as there were so few people coming in that direction."

Reunited at last, the family made their way back to their rented room. Once there, countless survivors who were so burned they appeared like "ghosts" to young Terao came streaming in, seeking their help.

The 4-year-old Terao recoiled into the corner of the room in fright. Shizuko -- though severely injured herself -- told her son she couldn't turn away people in need.

'Why do we still have these things?'

The next day, the boys and their mother tried again to find their former home, which was located just 300 meters (the length of three football fields) from ground zero. Back then, they did not realize they were putting themselves in further danger of radiation exposure.

"The house was burned, vaporized," Terao says. "My mom's best friends, acquaintances, nobody was alive. The only thing that survived from that area was our family. We thought we were lucky that we had survived."

The true extent of the damage from that day, however, is still being felt today. In the years that followed, both of Terao's brothers and his mother were diagnosed with cancers that they believe were linked to the radiation. While his brothers survived, his mother did not.

Now Terao looks at Ukraine and Russia and other rising security risks across the globe and worries for the world once again.

He notes that both China and North Korea have nuclear weapon programs and that Japan has proposed doubling its defense budget.

"Japan thinks it needs arms to protect the people. There is a dilemma. There is no easy answer," he concedes.

And yet, for a man who has survived an atomic bomb attack, the fact that the planet remains at risk of nuclear armaggedon is hard to live with.

"Why do we still have these things in the 21st century?" Terao asks.

"I wonder If I'll die without seeing a world without nuclear weapons," he adds. "I feel such shame when I think of that."