Detectives started their search of Ellowes Hall Wood in Lower Gornal today after 'further credible information' was received.
Around 20 police officers were involved in the search, which was initially focused on the pools in the Cotwall End Nature Reserve.
The search was expected to take two days – but a top West Midlands Police officer warned it could take weeks if a decision is taken to extend the search area.
It comes four months after officers exhumed graves at Ruiton Cemetery in search of Natalie's body.
Natalie disappeared from her home in Thornleigh 14 years ago. Officers are treating the case as a murder inquiry.
A detailed search is now being conducted in the woodland using specialist search techniques, including the underwater search unit.
Detective Superintendent Tom Chisholm, head of homicide, said: "This area was searched when Natalie went missing in August 2003 as part of the then missing person inquiry, but the intelligence picture has changed significantly since then.
"The information that we have received is corroborated by separate sources and is completely new information that was not previously known to us prior to June 2017.
"As such it is considered essential to the investigation that this information is thoroughly followed up in the search for Natalie’s remains."
Police said Natalie’s family have been fully updated with the latest developments and are being supported by specialist officers.
DS Chisholm said he believed people in the local area knew what had happened to Natalie.
He said: "We looked at some graves not too far from here earlier this year.
"That was on the back of community intelligence.
"During that search we received some more intelligence saying Natalie's remains would be down here.
"We're here to have a more detailed search of this area, which we did search at the time of her disappearance when Natalie was being treated as a missing person.
"Places like these woods would be a likely place for someone to bury a body."
DS Chisholm added: "I am convinced that there are people who still live in the area who hold the key to solving the disappearance of Natalie, I would urge those people to come forward.
"There are people who need to be mindful that there is an absolute determination from the police to find the answers concerning her disappearance.
"Over the years the person who is responsible for Natalie’s death will have confided in someone, I would urge that confidante to come forward."
Natalie left behind her 11-week-old son when she went missing in 2003.
She was 17 at the time of her disappearance – so would have been 31 today.