"We saw communities not too far from any coral reef, with plenty of fish, corals and action going on, some really striking individual colonies," she said.

The 15 megatonne hydrogen bomb was 1,000 times more powerful than the blast which destroyed Hiroshima, vapourising islands with temperatures hitting 55,000 Celsius (99,000 Fahrenheit), and shaking islands even up to 200 kms (124 miles) away.

The resulting 7km-wide fireball left a crater 2km across and 73m deep, while the mushroom cloud rose 100 kms over the South Pacific and radioactive fallout reached Australia and Japan.

Richards, from the Australian government-backed Centre of Excellence for Coral Reef Studies, said the research team from Germany, Italy, Hawaii, Australia and the Marshall Islands found corals up to 8 metres high and some with 30cm-thick trunks.

"It was fascinating. I've never seen corals growing like trees outside of the Marshall Islands," Richards said.

While above-water areas remained contaminated and unfit for human habitation, healthy sub-sea species probably travelled on strong winds and currents from nearby Rongelap Atoll, which was not bombed in a series of 23 tests between 1946-58.

"It is absolutely pristine for another tragic reason. It received fallout and was evacuated of people, so now underwater it's really healthy and prevailing winds have probably been seeding Bikini Atoll's recovery," Richards said.

Compared with a study made before the atomic tests, the team established that 42 species were missing compared to the early 1950s, with at least 28 of those locally extinct.

The team was asked by Marshall Islands authorities to investigate Bikini for the first time since the tests, in part to see if a small diving industry could safely be expanded.

The waters around Bikini are littered with wrecks of old, decommissioned ships sunk during the atomic tests, including the aircraft carrier USS Saratoga and the former Japanese flagship HIJMS Nagato, from which Admiral Yamoto gave the order to attack Pearl Harbour.

Richards said the ability of Bikini's corals to bounce back from "a single huge destructive event" was proof of their resilience, although that did not mean the threat to corals from climate change had been overestimated.

"Climate change is an ongoing struggle to survive with coral, with no reprieve in sight," she said. "After the atomic blasts they had 50 years undisturbed to recover."