The Aurora Borealis (i.e. the northern lights) is, along with its counterpart in the South Pole, Mother's Nature's greatest light show. Caused by electrically charged particles from the sun colliding with the Earth's atmosphere, the result is rippling curtains of green, pink, blue, and violet lighting up the night sky. Tour companies across the Arctic Circle offer light-hunting expeditions. You can opt for a single day, like this new trip from Off the Map that claims to be the most environmentally-friendly northern lights holiday in the world, or embark on a multi-day adventure to several locations, like this 11-day expedition through the Norwegian and Swedish Lapland from Holidays by Rail.