An underwater volcano is thought to be behind a huge column of steam above the Pacific Ocean, Japanese officials say.
The coast guard sent helicopters to monitor the 1,000m (3,280ft) cloud, 1,120km (700 miles) south-east of Tokyo, and warned ships to stay away.
The team said the area around the site appeared to be red.
"It's highly likely that it's caused by an eruption of an underwater volcano," coast guard spokesman Shigeyuki Sato said, adding it had happened before.
Comet Tempel 1 has smashed into the Deep Impact probe, producing a blast of light that prompted the mission control room at NASA's Jet Propulsion Laboratory in California, US, to erupt into cheers and applause.
Scientists and engineers jumped in the air, pumped their fists and hugged one another. Not only had their mission to deliberately collide with a comet for the first time succeeded perfectly, but the prospect of a damp squib - with the impactor passing right through a diffuse, rubbly comet - had fizzled away.
Astronomers as far back as Aristotle have speculated about the sudden appearance of comets that blaze through the night sky.
MIT scientists have brought a supercool end to a heated race among physicists: They have become the first to create a new type of matter, a gas of atoms that shows high-temperature superfluidity.
Experts have long known that the brain has the ability to suspend the pain response in times of injury and great stress, even after traumatic incidents such as gunshot wounds.
Now, a new study in rats suggests marijuana-like neurochemicals called endocannabinoids may be key to this process.
The discovery may lead to a new class of painkillers with fewer side effects than existing pain medications, report researchers at the University of California, Irvine (UCI). Their study appears in the June 23 issue of Nature.