Bhutan is arguably the world's happiest country. It's also one of the greenest. That's no coincidence. In fact, King Jigme Singye Wangchuck developed his signature Gross National Happiness index based on four pillars: sustainable development, environmental protection, cultural preservation, and good governance. Other countries have taken note, since the Himalayan kingdom is not only carbon neutral, but carbon negative. Also noteworthy: This is happening despite increasing tourism. As a travel destination Bhutan remains unique, sandwiched between its heavily industrialized neighbors China and India. The isolated nation only opened up to foreign visits in 1974 and allowed TVs in 1999. Bhutan has built sustainability into its national identity. In fact, the constitution mandates that 60 percent of its landmass be maintained and protected as forest. "The health facilities in Bhutan are free and education up to high school is also free. For those who advance, the education is free until the [college] degree," a representative of the Bhutan Tourism Council wrote in an email. There are many reasons Bhutan is carbon negative. Aside from its protected forests, it has won world records for planting the most trees per hour, says Erin Levi, the author of the forthcoming Bradt Travel Guide to Bhutan. "The ratio of people to land mass-it's about the same size as Switzerland with just one tenth the population. Its slow path to development-the first road was only built in the 1960s, which also means people were very slow to get cars," Levi said.