Video report by Tom Barton
Darren Blyth was last seen on Wednesday April 14 when he left his home in Trimdon Station. A keen naturalist, his wife Mel says he was going out to take photographs as it was a nice evening.
When he didn't return home, Mel went out to look for him, searching places they often visited together. Shortly after midnight she found his car abandoned at Hawthorn Dene in County Durham.
Durham Constabulary have said they are "increasingly concerned" about his welfare.
Ray Mears and his wife Ruth are family friends of the Blyth's. Ray has been using his topography skills to try and work out areas of interest to help focus the search, and distinguish which potential sightings might be likely.
CCTV footage and potential sightings
Darren was spotted on CCTV returning his camera to the car before walking off in the direction of Hawthorn Dene again.
Mel says there have been reports of potential sightings via social media channels, leading the search to move focus from County Durham to the North Yorkshire area.
Darren has been described as 5ft 10in tall with a slim build and short grey hair. He was last seen wearing a black North Face coat and grey outdoor trousers.
"He's my soulmate, he's priceless"
Darren’s wife, Mel, appealed directly to him to come home, writing him a letter in the hope that he will return safely.
She said: “To my darling husband Darren, if you are reading this then I want you to know that you are my entire world.
“You are the most beautiful, kind and generous soul I could ever wish to spend my life with, and nothing could ever replace my love for you. You are priceless.
“I will not rest until you return home safely to me, Abigail and your family who love you more than you could ever know.
You are truly the most important person in my life and I need you back safely with me. I know how much you love us, and that is why I know you want to come home.
“I can’t express how elated that would make me feel, so please, please, come home where we can make you feel safe, happy and loved.”
Darren is described as much-loved science teacher at Stokesley School in North Yorkshire. He has two daughters, nine-year-old Abigail and 36-year-old Katrina. He's been married to his wife Mel for 27 years.
She says her world has turned upside down since her husband went missing.
Katrina, Darren’s daughter, said: “Dad, the last thing you said to me was, ‘Thank you. It’s good to have people you can rely on.’
There isn’t a problem that we can’t solve together as a family. We can get through anything as long as we’re together.
“Mel, Abigail, Alex, Tamzin, Sean and I just want you home safe. Please get in touch to let us know that you’re okay.
“We love you so much.”
Anybody with any information relating to his whereabouts has been urged to contact police on 101, quoting reference number 005 of April 15.