Being outside with nature is where he felt most comfortable. That’s why Gary Michael Hilton targeted avid hikers in peaceful, wooded areas. But while he chose his victims carefully, he didn't give the same type of thought to getting away with his crimes.

On June 14th, 1969, 6-year-old Dennis Martin wandered away from his father in the Great Smoky Mountains National Park. He was never seen again. Since then, in the same area, there have been 4 other unsolved missing persons. Could they be connected? We'll find out...

Karen Silkwood was a chemical technician and labor union activist who raised concerns about a nuclear facility in Oklahoma. On November 13th, 1974, she was on the way to meet with a New York Times journalist when she died in a suspicious car accident. The files she allegedly planned to reveal were never found. And her car allegedly had signs of being run off the road. But nobody was ever arrested.

In 2016, Honduran environmentalist Berta Caceres was assassinated by a group of hitmen hired by an executive of a hydroelectric dam company.

For 18 years, she dedicated her life to the plight of the endangered gorilla. But just after Christmas of 1985, Dian Fossey lost her life — slain by a machete at her research camp in Rwanda. Publically, Fossey was known as a gentle conservationist. But those who knew her personally experienced her darker side. A ruthlessness that could have led to her death. But at whose hands?

On April 20th, 2010, BP's oil rig, Deepwater Horizon, exploded, killing 11 people. But the ensuing oil spill is still claiming victims. Researchers estimate 11-25% of oil from the spill remains unaccounted for. In a 130 million gallon spill, that’s 14 - 30 million gallons. For comparison, the entire Exxon Valdez disaster was 11 million. That's a lot of missing oil in the Gulf of Mexico. So, where did it all go?

Worker safety was paramount to Karen Silkwood. That's why she was proud to be part of the Oil, Chemical and Atomic Workers Union bargaining committee to renegotiate its contract with the Kerr-McGee nuclear plant. But before the meetings started, her car mysteriously went off the road, killing her.

Out in nature, alone with only the trees for company, Gary Michael Hilton found a peace that always eluded him in day-to-day life. Numerous run-ins with landlords, spouses, and law enforcement complicated things in the city. And in October of 2007, he went from petty thief to murderer.

Philadelphia’s first Earth Day celebration made him a minor celebrity. When he met 25-year-old Holly Maddux, he knew he had to be with her. They lived together for a few years before Holly started to see the real Ira Einhorn. Just days after she told others she was leaving him, she disappeared.

He dedicated most of his life to protecting rubber trees in the Amazonian town of Xapuri, where he lived. During that time, Chico Mendes received countless death threats. But on the night of December 22nd, 1988, somebody made good on their promise.

In one year, over 800 million people will suffer from hunger. Yet, in that same year, 1.3 billion tons of food will be discarded. Food waste is a global problem that can be solved. So, why hasn’t it been? And how long before its effects on our population and environment become irreversible?

By age 34, Dian Fossey was ready to start pursuing her passion. She knew her life’s work was in Africa, she just needed funding to get there. With the backing of famed anthropologist Louis Leakey, Fossey ultimately set up camp in Rwanda. Her groundbreaking studies on gorilla behaviors would make her a household name around the world. Her methods would make her an enemy of the locals.

Cities worldwide are headed for a "Day Zero'' when taps will run dry. Even though water covers over 70% of the Earth's surface, only about 1% is fresh enough to drink. Scientists are working on ways to desalinate water, but will it be enough, and if so, will it be in time?

If you were in Philadelphia during the 1960s and interested in the counterculture movement, you probably saw Ira Einhorn storm the stage at the city’s very first Earth Day. While peace and love were the mainstays of his belief system, Ira didn't always practice what he preached.

Born the daughter of English colonists in Kenya, Joan Root was a conservationist, activist, and Oscar-nominated documentary filmmaker. When she noticed that Lake Naivasha, home to her 88-acre estate, was suffering environmental damage, she took action. Unfortunately, unintended consequences made her a target.