Happy Earth Day, Adrenalists. For 43 years, this global holiday has been held to celebrate our natural planet and draw attention to many of the threats facing our environment. As climate change concerns have grown, so has support for the holiday, with over 1 billion people expected to participate in 192 countries this year.

We can thank Earth’s natural landscapes for fueling adrenaline-infused exploration, adventure, and dozens of sports from climbing to kayaking. This week, we’ll be honoring the planet with our Earth Week series, starting with today’s look at some of the last rainforests and jungles in the world.

Amazonia

Known as “the lungs of the earth,” Amazonia is more than just an eye-poppingly large mass of forest. Amazonia has come to symbolize the health of the planet and our battle to save it in the face of logging and global warming. Also called the Amazon Jungle, it is a damp, broad leaf forest spanning most of the Amazon Basin of South America. Spanish soldier Francisco de Orellana, who came from Europe in 1541, gave the river its name after reporting pitched battles with tribes of female warriors, whom he compared to the Amazons of Greek legend. The Amazon hosts an immense diversity of flora and fauna. In fact, it is the world’s richest and most varied biological reservoir, hosting several million species of insects, plants, birds and other organisms. Many of which are still undocumented by science, hence its reputation as a giant medicine cabinet that could yield all kinds of scientific discoveries.

One of the last mighty rainforests of the world, the Amazon covers an area of 2,300,000 square miles. Comprising about 40 percent of Brazil’s total area, it is bordered by the Guiana Highlands to the north, the Andes Mountain Ranges to the west, the Brazilian central plateau to the south, and the Atlantic Ocean to the east. In short, the Amazon is amazing – all the more so because of the various little-known and un-contacted tribes it contains. Then, there are the supremely intriguing wildlife standouts – the boas, toucans, jaguars, macaws and spider monkeys.

In the 20th century, however, Brazil’s exploding population settled vast areas of the rainforest. The Amazon then shrank radically, thanks to the colonizers’ clearance of the land to grab lumber and make grazing pasture and farmland. In the 90s the Brazilian government teamed up with various international bodies and began a campaign to protect chunks of the forest from human-caused carnage.

New Guinea

New Guinea is drenched in mystery. The wilderness frontier occupies an island of the eastern Malay Archipelago, in the western Pacific Ocean, north of Australia. New Guinea is bounded by the Pacific Ocean to the north and various other stretches of sea. The island is governmentally split into two thrumming chunks: the western half contains the Indonesian region of Papua (formerly Irian Jaya); and its eastern half hosts the main part of Papua New Guinea – an independent country since 1975. The world’s second largest island after Greenland, New Guinea is 1,500 miles long.

New Guinea’s shape is often likened to that of a bird of paradise. As the World Wildlife Fund (WWF) puts it, in New Guinea, “evolution runs amok.” Just as well – all the variety helps make the lush, volcano-packed zone so engrossing. Other wildlife wonders to look out for include tree kangaroos, the world’s largest pigeon, the Southern crowned pigeon, and the longest lizard, Salvadore’s monitor lizard. New Guinea does a convincing rendition of a lost world. In another inspired description, the WWF describes it as “a natural laboratory for new creatures.” Or, it is, for now. Inevitably, human depredation poses a threat to one of the last dazzling jungles of the world.

The Congo Basin

As applies to all the wilderness zones in this list, the Congo Basin drips mystique. Set astride the Equator in west-central Africa, the Congo Basin spans an area of over 1.3 million square miles. The basin houses the second largest rainforest of the world after our friend the Amazon. The forest region is bordered on either side by slabs of savanna (grassy parkland). The forest and savanna often merge in a beautiful mosaic. Ever eloquent, the WWF describes the whole zone as “the Green Heart of Africa.” Its forests, the organization adds, harbor indigenous peoples, gorillas, elephants “and a bewildering array of other amazing wildlife.” But for how long?

Thanks to human encroachment, between 1990 and 2000, some 35,000 square miles of forests were lost in Central Africa – an area about three times the size of Belgium. With its partner, the WWF is doing its best to stop the rot in one of the last rainforests of the world. The Congo represents one of the most important wilderness areas remaining on Earth. At 500 million acres, it is even more sprawling than the state of Alaska. The Congo has been home to humans for over 50,000 years, and supplies food, fresh water and shelter to over 75 million people. That is more than the entire population of the United Kingdom. Almost 150 distinct ethnic groups operate in the Congo. The zone’s Ba’Aka people are among the best-known examples of the hunter-gatherer lifestyle that dates back to the roots of our species. The natives’ livelihood is deeply intertwined with the forest and depends on its existence.

Borneo

Like New Guinea, hot and humid Borneo is one of the world’s great islands. It lies southeast of the Malay Peninsula in the Greater Sunda group of the Malay Archipelago. Borneo is bounded by the South China Sea and various other bodies of water. Borneo is carved up and run by several entities: Indonesia, Malaysia and the kingdom of Brunei – a tremendously ornate and spectacular separate state.

The island hosts one of the oldest rainforests of the world. Much of the land is laced by navigable rivers, which underpin commerce. But Borneo is deeply feral. Like the other jungles mentioned, Borneo has a reputation as a vivid example of evolution. Picture enormous variation spectacularly embodied by the world’s largest flower, the extremely eccentric rafflesia. Then there is the gibbon, the orangutan, the clouded leopard, elephant, rhinoceros, and a blizzard of insects. By Asian standards the island is thinly inhabited, still giving visitors plenty of room to roam. The island also has a history of headhunting executed by the Dayak people, who are probably best-known for that very act . Like all these last jungles of the world, Borneo is synonymous with adventure Watch out for its thunderously excellent white-water rafting stretch at Padas.