More than 30 countries around the Pacific Ocean have tested a system to warn them of approaching tsunamis.
The exercise began with a mock alert at the Tsunami Warning Centre in Hawaii.
An earthquake with a magnitude 9.2 was imagined to have struck near the coast of Chile, sending a tsunami racing across the eastern Pacific.
A second mock earthquake alert, north of the Philippines, will provide a further test on Wednesday.
Governments will report back on how efficiently they received the tsunami warnings, relayed through various circuits including weather services, emails and faxes.
The drill, co-ordinated by the Hawaii warning centre, will also measure how well the message is relayed through local emergency systems.
‘Already a success’
At the start of the test, a beeping noise sounded throughout the warning centre on Hawaii’s Ewa Beach, and within minutes the first alerts were sent out.
“Even before the exercise started, we considered it to be a success in the sense of having so much interest from all the countries,” said Charles McCreery, director of the warning centre.
“Showing their willingness to co-operate, that’s something we have never seen before.”
Correspondents say governments’ interest in tsunami alerts had waned before the catastrophic Indian Ocean tsunami of December 2004, which took more than 200,000 lives.
Two actual earthquakes struck on Tuesday during the test – a magnitude 7.4 quake north of New Zealand, and a magnitude 6.8 off Indonesia. No casualties were reported.
Another mock test on Wednesday will envisage a magnitude 8.8 earthquake north of the Philippines.
Officials there, and in Thailand, Malaysia and American Samoa, will go one step further by staging real evacuations.
A Pacific warning system has been in place since 1965, but this is largest test of the system since its inception.
The exercise may serve as a model for future tests in the Indian Ocean.