Africa's elephants are in crisis.
Over the past two years, Botswana-based elephant ecologist Dr Mike Chase has been counting the animals from a light plane.
He says the Great Elephant Census -- that is, a standardised survey of African elephants across the continent -- has never been conducted before.
"We spent thousands of hours in transit flying over areas where elephants historically occurred but are no longer present in these habitats. Some landscapes, we saw more dead elephants than live elephants."
During a routine flight, accompanied by a CNN news crew, Dr Chase landed for a closer look at one of the slain elephants.
He says he believes it had been killed just three days earlier.
"He was spectacular. Look at how big he was. In fact, not even three days (dead). And there you have clear evidence of his face hacked away like that. He met his end with people chopping away at his tusks. I don't think anyone in the world has seen the number of dead elephants that I've seen over the last few years at the Great Elephant Census, and, for me, this becomes a lot more personal."
Habitat loss and fragmentation, armed conflict and mining have affected the African elephant population.
But poaching is the biggest problem.
Since the early 2000s, the rise of the middle class in Asia has fuelled a new demand for ivory products.
The black-market wholesale price for ivory soared in China from $120 a kilogram in 2002 to more than $2,000 in 2014.
Environmentalist Sir David Attenborough has told CNN the possible extinction of African elephants is unacceptable.
"Oh, I think it's a crime, and a crime that, if it happens, will rest heavily on humanity's shoulders. What a dreadful thing to do. Appalling. And for what reason? For vanity, or for ... I mean, people believe that ivory is magical in some kind of way. Well, I can't argue about that, but it's not that magical that it justifies exterminating a wonderful, extraordinary animal like that."
Of the 18 countries surveyed, the biggest drops in numbers were recorded in Angola, Mozambique and Tanzania.
Botswana has more elephants than any other country in Africa, with a population of more than 130,000.
But the elephants are under constant threat.
More than 700 soldiers now guard the country's borders, and armed teams spend days on foot with a mandate to "shoot to kill" poachers.
The poachers are highly organised groups of about 12 people each, some with special-forces training.
Sir David Attenborough says the sale, or possession, of ivory must be outlawed around the globe.
"I think, in the end, it's going to have to come to it that they're going to become totally protected and that the sale of ivory -- certainly, of ivory collected or no more than 100 years old -- should be illegal. I don't see any way of getting around it."
African elephants are declining at a rate faster than they are being born.
About 30,000 are lost each year.
Experts say, if poaching is stopped, elephants might have a chance at regenerating populations.
The Wildlife Conservation Society's Simon Hedges has told the BBC enforcing anti-poaching laws must be better enforced across borders.
"There are strongholds -- Gabon, in central Africa, for example. And I think it just means that we need to refocus as a conservation community, that the governments in those countries and their conservation partners need to provide effective law enforcement at the key sites and do as much as we can to reduce trafficking in ivory and the demand for ivory in East Asia."
Despite the worrying findings, researchers say there are some signs of hope.
In a few countries, elephant populations are stable -- some even slightly increasing.
Herds have been found in Ethiopia and Kenya in areas where local populations were thought to be extinct.
But perhaps one of the nicest surprises was revealed in satellite data, showing the animals are avoiding places where they are in danger.
Dr Chase explains.
"You know, elephants clearly have a cognitive ability to understand where they are threatened and where they are safe, and, in this case, they are seeking refuge and sanctuary in Botswana -- where they're well-protected.
Researchers hope the data from the Great Elephant Census can guide policy decisions aimed at better protecting elephants.
