In the southern portion of China there is an expansive karst landscape, formed by the dissolution of soluble rocks such as limestone, dolomite, and gypsum. The region is home to the South China Karst UNESCO World Heritage Site, which is actually seven different notable features, as well as the visually impressive Moon Hill, some of China's supercaves, and Xiaozhai Tiankeng, the world's deepest sinkhole. You can climb Moon Hill, but it's best to plan ahead. You can also explore China's great caves, but it is necessary to explore between October-November and February-March to avoid the monsoon seasons, and getting down Xiaozhai Tiankeng requires a lot of gear. You can read more about the Tiankengs (giant dolines or sinkholes) in the karst of China (PDF).
Urban explorers surreptitiously gain access to the Post Office Railway underneath London, take lots of photos.
The Dzrtgrls explore mines, ghost towns, rockhounding spots, petroglyphs, geocaching and metal detecting sites, and take lots of great pictures in the process.
The United States Exploring Expedition, 1838-1842 — Authorized and funded by the U.S. government, six ships sailed with 346 men (including officers, crew, scientists, and artists) on a four-year scientific and surveying mission, logging 87,000 miles around the Atlantic, Pacific, and Indian oceans. Two ships and 28 men were lost, and the Expedition's contentious commander Charles Wilkes was court-martialled for his erratic behavior, and was sued by former officers and crew members. During the Civil War in 1861, he boarded a British ship, seized two Confederate agents, and nearly provoked military retaliation by England (he was court-martialled once again in 1864 for insubordination.) Wilkes' 1845 Narrative of the United States Exploring Expedition and the Ex. Ex.'s journals were published by Congress, and some 40 tons of Expedition specimens and artifacts became the foundation of the Smithsonian Institution's collections. [Nathaniel Philbrick (video lecture) chronicles this almost-forgotten voyage in his 2003 book Sea of Glory (NYT review).]
"The object of this organization shall be to explore the mountains, forests and water courses of the Pacific Northwest, and to gather into permanent form the history and traditions of this region; to preserve, by protective legislation or otherwise, the natural beauty of the Northwest coast of America; to make frequent or periodical expeditions into these regions in fulfillment of the above purposes. Finally, and above all, to encourage and promote the spirit of good fellowship and camaraderie among the lovers of out-door life in the West." Thus reads the charter of the Mountaineers. Explore the Tacoma Public Library's online exhibit of the Mountaineer's early history, Magnificent Views and Vistas, Mountaineer's Climbs 1912 to 1916.
Urban Exploring. Recently: Sanatorio Popolare Cantonale di Piotta. Sinteranlage, Duisburg. Atomschutz Kurfürstendamm, Berlin (flash). (Previously.)