All European airspace higher than 20,000 feet is open for flying today, a week after a massive volcanic ash cloud settled over the continent. But the airlines are physically and financially far from home free, according to scientists and industry officials.
"The best-case scenario is the worst is over, but I have my doubts," author and physicist Michio Kaku told "Good Morning America" today. "The more likely scenario is that this is a repeat of 1821, where for 14 months that very same volcano sputtered. We'll have to look for windows to shoot airplanes through or over.
"It's going to sputter for quite a while," Kaku said.
Least likely, the worst-case scenario, Kaku said, was that the eruption in Iceland could set off "Big Brother," referring to Iceland's Katla volcano, which Kaku said could be 10 times more powerful than the current eruption. Other scientists in Iceland told "GMA" further eruptions weren't likely to cause a similar ash cloud since this eruption occurred during unusual wind conditions and the ash was particularly fine, allowing it to be carried by the wind more easily.
The airline industry has already lost an estimated $1.7 billion, according to the International Air Transport Association. Eurocontrol, Europe's air safety organization, said the ash cloud caused the cancellation of more than 100,000 flights since last week, affecting millions of passengers every day.
In response, the IATI requested governments examine ways to compensate airlines for lost revenue, similar to how the U.S. government did following the terrorist attacks of Sept. 11, 2001.
"I am the first one to say that this industry does not want or need bailouts. But this crisis is not the result of running our business badly," said IATI Director General Giovanni Bisignani in a statement. "It is an extraordinary situation exaggerated with a poor decision-making process by national governments. The airlines could not do business normally. Governments should help carriers recover the cost of this disruption. ... This crisis is an act of God -- completely beyond the control of airlines."
If there are more ash cloud plumes, there is not much airplanes can do at the moment to deal with them, ABC News aviation consultant John Nance told "GMA."
"There's nothing that we're going to do to get airplanes to fly safely through ash clouds. ... We need better methods to find out where it is," Nance said. "Our radar can't pick this up. ... We were totally unprepared for this."
More than 75 percent of air traffic is expected to operate today, totaling an estimated 21,000 flights out of the usual 28,087, Eurocontrol said. But it could still be weeks before airlines catch up to the backlog of flights from earlier cancellations.
Last Major Hub, London's Heathrow, Reopens
The last major European hub to open, London's Heathrow airport, allowed its first flight to land Tuesday night.
Airports in Britain had stayed closed longer than other European hubs, due to the threat of more ash clouds blowing into British airspace.
The volcano in southern Iceland is still spewing smoke and lava, but the ash plume is lower than it previously was, posing less threat to high-flying aircraft. One of Iceland's top scientists, Magnus Gudmundsson of Iceland University did note, though, that scientists "cannot be sure" the situation will continue to improve.
Stranded Passengers Cheer Liftoffs
The sound of jets finally taking off was music to the ears of stranded passengers.
"We were in the hotel having breakfast, and we heard an aircraft take off. Everybody got up and applauded," said Bob Basso, 81, of San Diego, who has been staying in a hotel near Charles de Gaulle Airport near Paris since his flight Friday was canceled.
Ash Plume Is Lower, Easing Threat to High Flying Jets
Officials from across Europe agreed on Monday to create three zones for flying over Europe -- a no-fly zone over the ash cloud, a caution zone in nearby areas "with some contamination," and an open-skies zone. Planes flying in the caution zone will need to be checked for engine damage.
For those passengers lucky enough to be on the first flights out Monday, elation at finally getting in the air was only tempered by a fear of what the ash might do.
"I was very nervous," Lufthansa passenger Dominique Burkhard said of her Los Angeles to Frankfurt, Germany, flight. "One time we had turbulence. This was not so funny."
Britain Suspends 'Titanic' Laws to Help Ferries Carry More, U.S. Offers Emergency Loans
While it was still unable to take to the skies, the British government hoped to ease waterborne escapes from the island by suspending the "Titanic Laws," allowing ferries to carry more passengers than would normally be legal.
It's the first time such a measure has been taken since the laws' implementation nearly 90 years ago after the Titanic disaster in 1912. Dozens died after the massive ship's fateful run-in with an iceberg partially because there were far too many passengers for the number of lifeboats on the ship.
For Americans, home is a lot farther than a ferry ride away and the U.S. State Department said it had no plans to evacuate citizens by air or water, noting that by the time an evacuation could be organized, commercial flights would likely be available.
For those who have run out of money while stranded abroad, the State Department said it had a "limited amount of emergency loans that may be made available" under some circumstances and will help people send money to stranded loved ones.
ABC News' Maeva Bambuck, Bradley Blackburn, Kirit Radia and the Associated Press contributed to this report.