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For the new study, Danovaro and colleagues studied historical reports of mucilage in the Mediterranean from 1950 to 2008. Outbreaks, they discovered, were more likely when sea-surface temperatures were warmer than average.

Swimming Into "Mucus"

In 1991, Italian marine biologist Serena Fonda Umani swam alongside a mucilage—the mass is too dense to swim inside—in the Adriatic Sea, an arm of the Mediterranean (Adriatic Sea map).

She remembers diving about 50 feet (15 meters) down when she got the sensation of a ghost floating over her—"sort of an alien experience."

Umani, a co-author of the new study with Danovaro and Antonio Pusceddu, of the Polytechnic University of Marche, has also dived into marine snow—the mucilage's precursor.

She described it like swimming through a sugar solution. Out of the water, the dried "sugar" stiffened her hair and stuck to her wetsuit.

"The suit was impossible to wash totally, because it was covered by a layer of greenish slime," said Umani, of Italy's University of Trieste. "It was a nightmare."

Few people would purposely swim into a mucilage, said Farooq Azam, a marine microbiologist at the University of California's Scripps Institution of Oceanography.

"If you were not familiar with this—and especially if you were familiar—you wouldn't want to go near it," said Azam, who was not involved in the new study.

A giant odiferous blob drifting offshore is "certainly not the seascape that one goes to the beach [for]," Azam added.

Public Health Hazard

Eager to see if the blobs' side effects extend beyond ruined wetsuits, Umani and colleagues sampled coastal waters and mucilage from the Adriatic in 2007. The warm, shallow sea is like a "big bathtub," Scripps's Azam said—an ideal natural laboratory for studying the blobs.

The study team discovered that the blobs are hot spots for viruses and bacteria, including the deadly E. coli. Coastal communities regularly test for E. coli, and its presence is enough to close beaches to swimming.

Study leader Donavaro said, "Now we see that … the release of pathogens from the mucilage can be potentially problematic" for human health.

(Related: "Beach Bacteria Warning: That Sand May Be Contaminated.")

People who swim through mucilage can also develop skin conditions such as dermatitis, he added.

Suffocated by Blobs

Fish and other marine animals that have no choice but to swim with mucilages are most vulnerable to their disease-carrying bacteria, which can kill even large fish, the study says.

(Quiz: Test your ocean IQ.)

The noxious masses can also trap animals, coating their gills and suffocating them, Danovaro said.

And the biggest blobs can sink to the bottom, acting like a huge blanket that smothers life on the seafloor.

Mucilages Going Global?

Mucilages aren't a concern for just the Mediterranean, Danovaro added. Recent studies tentatively suggest that mucus may be spreading throughout oceans from the North Sea (map) to Australia, perhaps because of rising temperatures, he said.

"It's a good example [of what will happen if] we don't do something to stop climate warming," Danovaro said. "There are consequences [if] we continue to deny the scientific evidence."

Beyond warm temperatures, it's still not exactly clear what drives the blobs' formation, Scripps' Azam pointed out. For instance, no one knows why the dead marine matter in the blobs doesn't decompose.

"It's important we do find out" what's driving the rise of the blobs, Azam said, "for the sake of the rest of the worlds' oceans."

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