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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Stephen Kinnock has led a delegation of campaigners calling for a posthumous pardon for Dic Penderyn.
The latest figures show skin cancer numbers for people over 55 have almost tripled in the last 20 years.
The number of people diagnosed with the most dangerous form of skin cancer has almost tripled in 20 years, according to new figures.