More than 30 countries around the Pacific Ocean have tested a system to warn them of approaching tsunamis.
Scientists were pleased with the high participation in the test
The exercise began with a mock alert at the Tsunami Warning Centre in Hawaii.
According to the scenario, a magnitude 9.2 earthquake had struck near the coast of Chile, sending a tsunami racing across the eastern Pacific.
A second mock alert, involving a make-believe earthquake north of the Philippines, has been testing responses in the western Pacific.
The drill is thought to have been broadly successful, although there was some delays with communications systems in Thailand.
Governments are reporting back on how efficiently they received the tsunami warnings, relayed through various circuits including weather services, emails and faxes.
The aim of the drill, co-ordinated by the Pacific Tsunami Warning Centre in Hawaii, is also to measure how well the message is relayed through local emergency systems.
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 to 30 participating countries.
In a second part of the drill, officials in Thailand, Malaysia, American Samoa and the Philippines staged real tsunami evacuations.
On Wednesday morning, a mock warning of an earthquake north of the Philippines sent nearly 1,000 people in the coastal village of Buhatan scurrying for the hills.
In Malaysia, villages along the coast of Sabah state on Borneo were also evacuated as part of the drill.
"It's gone very, very well so far," a spokeswoman for the US National Oceanic and Atmospheric Administration told AFP news agency at the end of the first stage.
"They've contacted each country that is participating and just about every single one of them have received the bulletins," Delores Clark added.
The second part was not quite as smooth. A crucial link in the communications chain to alert people in Thailand failed to work, a disaster response official said.
The country's National Disaster Warning Centre said the problem was caused by an overloaded telephone system which delayed public text message alerts for several hours.
"This is something we need to improve, otherwise it may cause great damage," Samith Dhammasaroj, the head of the centre, told Reuters news agency.
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.
Some countries planned evacuations at coastal areas
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 is envisaging 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.