Standing on high ground overlooking the town of Machynlleth puts into perspective how big a search this is.
Teams could be seen in the distance, their fluorescent jackets standing out like beacons in the fields.
A row of five men walked slowly, heads stooped, clutching sticks, as they combed a patch of land alongside the river.
Another man with a dog scoured the opposite riverbank.
And further up were five green canoes moving slowly against the fast flow of this flooded river.
Their task is huge. And even though the weather was fair and sunny today, as each hour passes it is a task which gets harder and harder.
They are looking for five-year-old April Jones but they have not found her.
The smallest of details could mean the biggest of breakthroughs.
I spent the day with Perry McGee, a tracker who specialises in finding people in hostile environments. He says the search teams have an "enormous job".
"It's tremendously difficult. My heart goes out to everybody doing their upmost," he said.
"The weather, the terrain, the resources, everything has been against them.
"Yet they still manage to pull out volunteers from the hat all through the night all through the day. They're doing a tremendous job."
He showed me how trackers are able to spot tell-tale signs that people have passed through an area. It could help find April, he says.
"Tracking is not just about looking at what has been physically disturbed in an area, it's about smell, hearing and observing.
"Fence posts, loose barbed wire and footprints in the mud can tell us where somebody has gone, their direction, their weight, whether they are male or female.
"The position of broken cobwebs could indicate where somebody has been and the direction in which they were travelling.
"But the key thing, especially in the case of a lost girl, is the basics.
"The Americans talk about the rule of three; three days without water, three days without food and three weeks without hope.
"If April has been here for three days then she could have serious injuries.
"And that's the desperate worry. Temperatures are expected to drop to just a few degrees above zero. No place for a little girl."
As we make our way down to the river we find David Jones. He has more than 40 years' experience in search and rescue.
"I'm doing bank search with water-search dogs going from the bridge and criss-crossing over and ready to do the other side as well," he explained.
Search teams returned to the command point at the local leisure centre as light began to fade over Machynlleth.
They looked exhausted. Their clothes were thick with mud and their boots soaked with rainwater. But they grab a coffee and a biscuit and they head straight back out again.