Editors note: The images below may be upsetting to some readers.

When photographer Paul Hilton arrived at a clearing in Sumatra, Indonesia, last year, there was just a backhoe, a couple of law enforcement officials and an open pit about to be set on fire the final act of a major wildlife crime bust.

The day before, Hilton had been in a warehouse in the city of Medan when police raided the building on a tip that a seafood trading company was trafficking in smuggled creatures hunted for their valuable body parts. He photographed the raid and the lucky critters that made it out alive. Though he found it heartbreaking, visually, it wasnt that impactful, Hilton said.

But it was the pit a few miles away that made it hard to lift his camera. The warehouse had been hiding boxes, crates and shipping containers full of frozen pangolins, the worlds most trafficked animal , destined for consumption abroad.

The trench Hilton strode up to that day was a mass grave, filled with nearly 4,000 pangolins that the authorities had seized from the seafood company.

I stood there for a while and I couldnt even take a photograph, said Hilton, who was on assignment for the Wildlife Conservation Society at the time. It was the early morning, a very stark landscape with just a few police officers. And Im just standing there looking into this pit thinking, What an absolute disgrace.