By Mike Chilvers
BBC News in East Yorkshire
The Valentine's Day disappearance of Joanne Nelson sparked what has now become the biggest search operation ever carried out by Humberside Police.
The main search areas stretch from North Lincolnshire to North Yorkshire
Hundreds of officers, search and rescue volunteers, police divers, coastguards and Army personnel have worked together in the hunt for the 22-year-old's body.
They have combed 800 sq km of often difficult terrain across four counties.
"The scale of this operation is unprecedented for this area," said force spokesman Insp Steve Page.
Miss Nelson, from Hull, disappeared on 14 February. Since then between 20 and 30 officers each day have been split into separate teams of up to eight people sent to scour specific areas on the orders of search co-ordinators Sgt Dave Scott and Sgt Colin Bone.
"The search teams are highly trained in a variety of search techniques, including searching for bodies," said Mr Page.
"They are all working very long hours in difficult terrain and at times appalling weather.
"They have done an extraordinary job covering huge tracts of land."
Joanne Nelson was reported missing on Valentine's Day
For several days, heavy snow and blizzards forced planned searches on high and exposed ground to be put back until the weather improved.
During the month-long operation, police have been assisted by hundreds of volunteers, including search and rescue teams from the Trossachs in Scotland, Scarborough in North Yorkshire and Louth in Lincolnshire, as well as International Rescue.
Police underwater search units, the Marine and Coastguard Agency, Territorial Army, special constables and police support staff volunteers have also been involved.
The Humberside Police helicopter equipped with thermal imaging cameras has been deployed, as have sniffer dogs trained specifically to detect bodies.
'Out of character'
The parameters of the search have been dictated by patchy intelligence passed to the search co-ordinators by the major incident team which was set up as soon as police realised Joanne's disappearance was "wholly out of character".
Detectives have been told only that the suspected murder victim's body is believed to be hidden under the branches of a pine tree close to a wood and a track leading to a metal gate.
This intelligence is "married up" with information from the public, such as possible sightings of Miss Nelson's car, as officers try to build up a picture of the locations they need to focus on.
Police divers have scoured waterways throughout the region
The senior investigating officer, Det Supt Ray Higgins, says the response from the public has been "outstanding".
"But that in itself causes problems because of the sheer volume of information which the team is having to sift through," Mr Page said.
Officers leading the investigation are using HOLMES (Home Office Large Major Enquiry System), the computer programme which all forces employ for complex cases where large amounts of data have to be processed.
Although they remain "hopeful" of finding Miss Nelson's body, officers concede that as time goes by their task is becoming more and more difficult.
"The longer a body is left exposed to the elements the harder it is to find and the more difficult it becomes to glean evidence from if it is found," said Mr Page.
"We are hoping to get more intelligence to help us narrow down the search area.
"The operation is being reviewed on a daily basis, but no time limit has been set down.
"When it is a case of finding somebody to help a devastated family cope with what has happened you just have to keep on going."