Iceland's Eyjafjallajokull's crater is only a mile and a half wide, but the ripple effect of its massive ash cloud is reverberating worldwide and getting worse, experts said.
"This could potentially be a problem for weeks to even months," Charles Mandeville, a volcanologist at New York's American Museum of Natural History, said.
Officials at Eurocontrol, the European air traffic agency, said that of the 29,000 flights that would normally head through European airspace today, only 12,000 to 13,000 will take place and delays are expected to continue at least into Saturday.
Eurocontrol announced that the airspace in nine countries is "currently not available" as well as airspace around major airports in France, Germany and Poland. According to the Polish press office, the funeral for the country's late president might be delayed.
"This is most significant air traffic control event since Sept. 11, and certainly the most significant that's ever hit all of Europe at one time," Brent Bowen, the head of Purdue University's aviation technology department, said Thursday.
According to ABC News aviation consultant John Nance, the cloud is not a direct threat to domestic U.S. flights but has already had an impact on fleet planning.
"This stuff will go all around the world, but by the time it gets back to the West Coast of the United States, now it's pretty well fallen out to the point where it's not a danger to us," Nance told "Good Morning America" today. "But we've never seen [such a volcanic disruption] on a scale like this where it has hit a tremendous beehive of daily aviation activity."
It is not yet clear when the flying restrictions will be lifted, but the Eyjafjallajokull volcano is still erupting and could continue spewing ash into the atmosphere for weeks.
"It is likely that the production of ash will continue at a comparable level for some days or weeks," said Einar Kjartansson, a geophysicist at the Icelandic Meteorological Office. "But where it disrupts travel, that depends on the weather."
British media outlets reported Thursday that Eurostar trains between London and Paris and Brussels are sold out.
"You can't get on a train out of England through the Chunnel now to mainland Europe. The ferries are filling up. People are trying to go over to Paris before it gets shut down. Amsterdam is already being affected. Copenhagen is being affected," Bowen said. "This is just having a continuous ripple effect."
Expert: Volcanic Ash Result of Perfect Storm of Conditions
The volcano at Eyjafjallajokull began erupting Wednesday, and both weather conditions on that day and the nature of the volcano made it just the wrong time.
The volcano is covered by a glacier, a big problem for eruptions, Mandeville said.
"Normally in Icelandic volcanic eruptions, the ash doesn't go this high because the gases come out easily, but in this case we have magma interacting with glacial melt water quite explosively, which lifts the ash to great heights," he said.
Meanwhile, winds over Iceland were in just the right place to carry the ash over Europe. The eruption launched a torrent of particles that are each 2 millimeters thick, but these hot, tiny pieces of rock, glass and sand taken together and sucked into a jet engine can melt and cause that engine to fail.
In 1982, a British Airways flight lost all four engines after flying through an ash cloud and got power back only after diving to a lower altitude. In 1989, Alaska's Mount Redoubt damaged a Boeing 747, but it, too, was able to land safely.
Even though the ash cloud is large enough to be seen from space, Nance said it's impossible for aviators to pinpoint where it is to avoid it.
"We don't have the tech to be able to know exactly where it is. Even when we're looking at it those heavy concentrations are not only deadly, they're five times deadly," he said. "You can fly through an awful lot of this and destructive portions of it without even seeing it.
"The only possible way of handling this is avoid it, avoid it, avoid it," he said.
ABC News' Lama Hasan and The Associated Press contributed to this report.