Major explorations continued after the Age of Discovery. By the early seventeenth century, vessels were sufficiently well built and their navigators competent enough to travel to virtually anywhere on the planet by sea. In the 17th century Dutch explorers such as Willem Jansz and Abel Tasman explored the coasts of Australia. European naval exploration mapped the Western and northern coasts of Australia, but the east coast had to wait for over a century, while in the eighteenth century it was English explorer James Cook who mapped much of Polynesia. Cook traveled as far as Alaska. In the later 18th century the Pacific became a focus of renewed interest, with northern Europeans exploring its last unknown regions and the North-American west coast. The centers of the Americas had been reached by the mid 16th century, although there were unexplored areas until the 18th and 19th centuries. Australia's and Africa's deep interiors were not explored by Europeans until the mid to late 19th and early 20th centuries; this being due to a lack of trade potential in this region, and to serious problems with contagious tropical diseases in sub-Saharan Africa's case. Finally, Antarctica's interior was explored, with North and South Poles reached in the 20th century.