Japan's nuclear crisis looks to be under control for now, with a total meltdown at the damaged Fukushima nuclear plant no longer likely. But the country's exhausted authorities are now facing a fresh problem — rising levels of radiation in the seawater off the coast of northern Japan. Here, a quick guide to what could be Japan's next threat:

How radioactive is the seawater?
Extremely so, at least in the immediate vicinity of the Fukushima nuclear plant. The Tokyo Electric Power Company found that the level of radioactive iodine in water 1,100 feet from the plant was 126 times the legal amount, and cesium levels were 24 times higher than normal. Water tested some 10 miles away from the plant contained up to 16 times the legal level of radioactive elements.

How long will the water be radioactive?
Scientists say the radioactive iodine will break down naturally in the water, but cesium tends to linger for longer — and spread wider. After Chernobyl, the element cesium 137 was detected in wild animals as far away as Croatia and Norway, some 15 years after the meltdown. Scientists worry that cesium could get into the food chain of Japan's marine life, and stay there for years.

What would be the impact on Japan's fishing industry?
It could be enormous. Tokyo's $5.4 billion fish market is the world's largest, and it's already reeling in the aftermath of the disaster. Many fishing centers in the north of the country were all but wiped out by the quake and tsunami. Now, customers are refusing to buy fish from anywhere in the region, due to potential contamination.

What do the authorities say?
They're trying to put a brave face on it. "There are no fish coming from the regions that were hit, so no fish are contaminated," said a spokesperson for the National Federation of Fisheries Cooperative Associations. "These kinds of rumors are the worst thing that could happen."

Are other food sources contaminated?
Yes. Four northern prefectures were forced to stop selling spinach and milk after high levels of radioactive iodine were detected. Although iodine decays quickly, it can be very harmful to humans and animals as it concentrates in the thyroid gland. Children who drank contaminated milk in the aftermath of the Chernobyl disaster later contracted an unusually high rate of thyroid cancer.

Sources: Christian Science Monitor, Wall St. Journal, New York Times, Reuters