The Indian Ocean tsunami in 2004 was not the first of its size to hit the region, according to new research.
Two international collaborations have sampled the sediments in Thailand and Sumatra to examine tsunami history.
At both sites, there was evidence of sediment laid down by a large tsunami between 600 and 700 years ago, pre-dating written and oral records.
The findings, reported in Nature, could be used to put statistical weight behind estimates of future tsunami.
The surge of a tsunami brings with it a great deal of sediment that rushes inland; the bigger the tsunami, the deeper and further inland the layer of sediment it leaves behind.
In locations where those deposits aren't disturbed by wind or running water, they can be used as a historical record of these powerful events after more layers are added.