Anaximander of Miletus
Anaximander lived between 612 and 545 B.C. He made the first attempt to draw a map of the inhabited world on a tablet and was renowned for explaining geographical phenomenon in a rational and non-mythological way. Anaximander is also credited with introducing Greece to the gnomon, an early form of sundial that assisted with determining equinoxes and solstices.
Alexander von Humboldt
Charles Darwin described Humboldt as "the greatest scientific traveler who ever lived." While exploring South America in the early 19th century, Humboldt measured and discovered the Peruvian Current. In 1827 he became the King of Prussia's adviser. His subsequent explorations of Russia led to his discovery of permafrost and his recommendation that weather observatories be established across the country. With these established, Humboldt developed his principle of continuity, stating that because the interiors of continents are farther from the ocean, they have more extreme climates. He also developed the world's first isotherm map.
Willem Barents
Between 1594 and 1597 William Barents made three voyages to try to find a Northeast Passage through the Arctic. He discovered the island of Spitsbergen but became trapped by sea ice during his final voyage in 1596. Barents and his crew were forced to spend the winter ashore, on the islands of Novaya Zemlya. In doing so, they became the first Europeans to survive a winter in the high Arctic.
Sir Henry Morton Stanley
Stanley was born in 1841 in Denbigh in North Wales as John Rowlands. He changed his name when he was 17 after sailing to New Orleans. Aside from finding the wayward Dr. Livingstone, Stanley's notable achievements include circumnavigating Victoria Nyanza, thus proving it to be the world's second-largest freshwater lake, and discovering the Shimeeyu River. He also made the first complete traverse of the Iruri River.