The Oceans. 72% of the planet's surface is covered by the ocean. That's roughly 140 million square miles of water with an average depth of roughly 12,500 feet, although some places like the Challenger Deep section of the Marianas Trench can reach nearly 36,000 feet, which is deeper than Mt. Everest is tall! There are currently around 230,000 aquatic species accounted for, although some estimates put the total number of unique aquatic organisms at more than 2,000,000 based on the amount of ocean we have left to explore. Just imagine what those murky depths might conceal!

Caves. Like the oceans, underground caves provide an untold number of possibilities just due to the sheer volume of space yet to be explored. While there are no definite estimates of how much cave space there is to explore, some things to keep in mind: Mammoth Cave in the United States has the longest recorded length of any cave system at nearly 400 miles; Voronya Cave in Georgia (the country) is the deepest at more than 7,000 feet; the largest single cave room yet discovered is in Malaysia and measures nearly half a mile long by a quarter-mile wide, with a ceiling height of 260 feet.

The Amazon Rainforest. The Amazon is so large it contains nearly 50% of the world's rainforest region. Its 2.1 million square miles includes land belonging to nine separate nations, and is largely unexplored due to the winding waterways and dense foliage. The Amazon is home to 16,000 different types of trees, 2.5 million insect species, 2,000 birds species, and over 2,200 fish species. Deforestation is a major concern; as it continues, who knows what we might lose - or find - in the process?

Deserts. Taking up about 1/3 of the Earth's land area, deserts remain largely unexplored due to their inhospitable living conditions. But those are some pretty big unexplored areas: the Sahara is 3.5 million square miles; the Arabian desert is 1 million. Even "small" (in comparison) deserts like the Great Basin Desert in North America span nearly 200,000 square miles. Plants and animals living in the extreme temperatures of desert regions have developed some extreme and even bizarre adaptations to the environment. Who knows what else has learned to adapt out there?

Antarctica. 5.4 million square miles of ice at the bottom of our planet. Temperatures can reach -129 degrees Fahrenheit. No permanent human residents. Since 1959, 49 countries have signed a treaty to leave Antarctica an undisturbed place of scientific research; military activity, waste dumping, and nuclear testing are all forbidden. There are a number of subglacial features underneath the icy surface of the continent, including Lake Vostok, the largest of Antarctica's 400+ underground lakes and larger than Lake Michigan. Who knows what lurks in those underground features? Or what designs those other countries that didn't sign the treaty have for our icy seventh continent?