Officers using specialist equipment are examining land around the Cariad Farm site in Marshfield, Newport.
The search - prompted by suspicion a body has been buried there - is expected to last for several days.
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Five young sailors from South Wales will be training in Pwllheli this morning ahead of the Optimist European Championships.
Wildlife experts have warned over the possible extinction of water voles, with a dramatic decline in numbers over the past 45 years.
A Wales Bill paving the way for the transfer of new powers to the Assembly is due to be amongst the new laws set out in the Queen's speech