Police investigating the 1996 disappearance of Melanie Hall have been searching several sites in the Bath area, following the arrest of two men in connection with the case.
Miss Hall has never been found
Miss Hall, then 25, from Bradford-on-Avon, Wiltshire, disappeared in the early hours of Sunday 9 June after a night out at Cadillacs nightclub in Bath.
The hospital clerical worker has never been found.
A team of officers are concentrating their search on a field near Inglesbatch, used as a motorcyle track.
Giant sieves were used during the search
A flat above a charity shop in Bath has also being checked, as has the Inglesbatch home of one of the men arrested on Wednesday.
A small area at the top of a sloping field at Inglesbatch, located in winding country lanes, was cordoned off while officers searched through the mud.
An Avon and Somerset police spokesman said the search could take some time.
Detective Inspector Ian Glover said: "It has to be done professionally and it may take some days."
"My officers are currently looking for clues or evidence relating to the investigation."
Inglesbatch is four miles from Bath and police are using Beaufort Farm as a base.
Officers used giant sieves on Thursday to search for clues in earth dug from a mound at the top of the field.
Water was used to flush the soil through as minute pieces were removed and kept for examination by forensic experts.
Police say they were directed to the field by a member of the public.
They have confirmed that neither of the men arrested was Miss Hall's boyfriend.
An incident room has been set up with 40 officers at Kingswood Police Station.
The two men arrested were released on police bail on Thursday.