British journalist Dom Phillips and Brazilian Indigenous specialist Bruno Pereira went missing on Sunday deep in the Amazon after receiving threats, in a remote region that houses some of the wildest jungle on Earth, home to an estimated 19 uncontacted Indigenous groups, as rights groups and families pleaded for authorities to accelerate the search operation.

Here are some fast facts on Brazil's Javari Valley, an immense Indigenous reservation near the border with Peru whose isolation makes it a haven for drug traffickers and prey to illegal logging, gold mining and poaching.

Bigger than Austria

Established in 2001, the Javari Valley Indigenous Reservation covers 85,000 square kilometers (33,000 square miles) – bigger than Austria.

Located in the northern state of Amazonas along Brazil's border with Peru, it is home to an estimated 6,300 Indigenous inhabitants.

It is the second-biggest Indigenous reservation in Brazil, after the Yanomami territory to the northeast, which spans 96,000 square kilometers.

Indigenous refuge

Twenty-six indigenous groups are believed to live on the reservation, including 19 that have little or no contact with the outside world.

It is one of the last refuges for such groups.

"The Javari Valley is home to more uncontacted tribes than anywhere else on Earth," says Indigenous rights group Survival International.

Deep jungle

It is a famously far-flung wilderness.

"You're talking about dense tropical forest," Fiona Watson, research director at Survival, told Agence France-Presse (AFP).

"It's all rainforest, with a lot of rivers running through it."

She recalled getting stranded in the region in the 1990s when her group's boat ran out of fuel.

"We just had to float down the river. But it would have taken days, because of the meandering curves of the river," she said.

"So one of our Indigenous guides decided to cut across as the bird flies, crossing the meanders."

He eventually made it back with a container of fuel.

But the region is notoriously difficult, she said.

"The operation to try and locate Bruno and Dom is immensely challenging."

Target for invasions

That remoteness has made the region attractive for drug traffickers, who have profited from the lack of state presence and porous border between two key countries in the narcotics trade.

The territory has also seen a "huge" increase in illegal logging, gold mining and poaching in recent years, Survival said.

"The land invasions and violence associated with this illegal activity poses a grave threat," it said.

The fact the disappearance occurred against that backdrop "makes the situation even more troubling," said the World Wildlife Fund (WWF) Brazil office.

Patrols, conflicts

Pereira, the former head of programs for uncontacted groups at Brazil's Indigenous affairs agency, FUNAI, has spent much of his career fighting such invasions – making him a target of frequent threats.

He had helped local Indigenous communities coordinate patrols of their land and was on his way to a meeting with a local community leader about the project when he and Phillips disappeared, according to local Indigenous activists.

FUNAI's base in the region, set up to protect Indigenous inhabitants, has come under attack several times in recent years.

In 2019, a FUNAI officer there was shot dead.