The first of a planned network of tsunami early warning buoys is being laid in the Indian Ocean.
The buoy is being placed between Thailand and Sri Lanka, two of the countries worst-hit by the 2004 tsunami which killed more than 200,000 people.
The buoy, provided by the US, is able to detect sudden increases in pressure deep under the sea and give coastal communities early warning of a tsunami.
The US already operates a similar system in the Pacific Ocean.
The cost of the US-designed device is being shared by the US and Thailand.