The company behind a bid to build a new nuclear power station on Anglesey is to carry out tests to assess the rock on the sea bed near the site.
Depending on the weather, Horizon Nuclear Power begin drilling this week.
A spokesman said the work was important as part of the studies the firm is carrying out near the site.
The new plant would be built to replace the existing Wylfa nuclear power station, which is due to be decommissioned in 2012.
The two year extension to electricity production was announced last month.
The Magnox-type reactors had been due to shut down in December after 39 years.