Tucked under the hillside of the lush forests of Alberta, Canada, is a huge grave.

Thousands of dinosaurs were buried here and killed in a day of complete destruction.

Now, a group of paleontologists come to Pipestone Creek (nicknamed "The River of Death") to help solve a mystery of 72 million people: how did they die?

Trying to figure out exactly what’s going on here starts with the huge strike of the sledgehammer.

Brute force was needed to open up the thick layer of rock, covering what Professor Emily Bamforth, who led the excavation, called "ancient gold".

As her team began to work on removing dirt and dust, a pile of fossil bones began to appear gradually.

"We think the big spots of bones are part of the buttocks," Professor Bamforth said.

"Then we have all these long bones. These are ribs. It's a neat bone - it's part of the toe bone. It's part of a toe. Here, we don't know what it is - it's a great example of the mystery of Pipe Creek."

BBC News has come to Pipestone Creek to witness the sheer scale of this prehistoric cemetery and learn how researchers piece together clues.

Thousands of fossils have been collected from the site and new discoveries are constantly being made.

These bones all belong to a dinosaur called pachyrhinosaurus. The species and Professor Bamforth's excavation work have a new landmark BBC series - Walking with Dinosaurs - which uses visual effects and science to bring this prehistoric world to life.

These animals that lived in the late Cretaceous were relatives of Triceratops. The four-legged beast is about five meters long and weighs 2 tons, has a large head, decorated with unique bone folds and three corners. Their decisive feature is the big bump on the nose, called the boss.

The excavation season has just begun and lasts until autumn every year. A small piece of ground that the team was dealing with was crowded with people. Professor Bamforth estimates that there are up to 300 bones per square meter.

Her team has so far digged a tennis court-sized area, but the bone bed extends for a kilometer.

“It’s getting a drop in density,” she told us.

“We believe this is one of the largest bone beds in North America.

"A from a specimen describes more than half of the world's known dinosaur species. We have thousands of Pachyrhinosaurus here."

Paleontologists believe that dinosaurs migrate together in a huge herd of cattle, crossing hundreds of miles in the south (where they spent the winter).

The climate of the region is much warmer than it is today and it will be covered with abundant vegetation, providing abundant food for this group of carnivores.

"It is a single community of single animals from snapshots, and it is a huge sample size. It almost never happens in fossil records," Professor Bamforth said.

Moreover, this patch in northwestern Alberta is not only the site of Pachyrhinosaurus. Larger dinosaurs roamed the land, and studying them was essential to try to understand this ancient ecosystem.

After two hours of driving, we reached the Death Hill. Getting there involves hiking in dense forests, wading in the space – or wandering in the green river – along a fast-running river and climbing up on slippery rocks.

There is no need to dig here; the oversized bones are located next to the shoreline, washed away from the rocks and washed by flowing water, just waiting to be picked up.

A huge vertebrae was soon discovered, and the ribs and teeth scattered on the mud were also quickly.

Paleontologist Jackson Sweder is particularly interested in what looks like a dinosaur skull. "Most of what we find here are duck-billed dinosaurs called Edmontosaurus. If this is a skull bone, then this is a big dinosaur - about 30 feet (10m)."

Another herbivores, Edmontosaurus, roamed the forest like Pachyrhinosaurus and was helping paleontologists build photos of the ancient land.

Sweden is the collection manager of the Philip J Currie Dinosaur Museum in the nearby Grande Prairie, where the bones of these two giants are considered cleansing and analyzing. He is currently making a huge pachyrhinosaurus skull of about 1.5 million, nicknamed "Big Sam".

He pointed out that the three corners should be at the top of the frills, but the middle corner is lost. "All the complete skulls have peaks in that place," he said. "But its nice little unicorn spike doesn't seem to exist."

Over the years working in an extraordinary location, the museum team has collected 8,000 dinosaur bones, and the surface of the laboratory is covered with fossils. Pachyrhinosaurus from young to old have bones.

Having materials from so many animals allows researchers to understand dinosaur biology and answer questions about how species grow and community pose. They can also look at individual changes and see how a Pachyrhinosaurus stands out from the herd - like Big Sam and his missing spike.

In the museum and two locations, all this detailed research is helping the team answer a crucial question: So many animals in Pipestone Creek die at the same time?

Professor Bamforth said: "We believe it is a seasonal herd of cattle that is tangled in some catastrophic events that effectively eliminate, if not the entire herd, a large part of it."

All evidence suggests that this catastrophic event was a flash flood - perhaps a storm in the mountains, causing unstoppable water to the herd, tearing off the trees from the roots, and transferring the boulders.

Professor Bamforth said Pachyrhinosaurus would not have a chance. “These animals can’t run very quickly due to their huge numbers, and they are very heavy – and really not very good at swimming.”

Rocks found at the site show fast-flowing water vortexes from the agitation of sediment in all objects. It seems that the destruction freezes with the tide of stones.

But for paleontologists, dinosaur nightmare is a dream.

Professor Bamforth said: "We know that every time we come here, there is a 100% guarantee that we will find bones. Every year we find new things about the species."

“That’s why we keep coming back because we’re still looking for something new.”

As the team is ready to return the tools, they know there is still much work in the future. They just scratched the surface here - and there were more prehistoric secrets waiting to be revealed.

The new Walking Series with Dinosaurs begins at BBC One's Sunday, May 25th BST at 18:25 BST, with all episodes available on BBC Iplayer.