Decken explored in the region of Lake Nyasa on his first expedition in 1860. The following year, together with a geologist, he visited the Kilimanjaro massif. Returning in 1862, he ascended Kilimanjaro to about 13,780 feet (4,200 m), observed its permanent snowcap, established its height at about 20,000 feet (6,100 m), and mapped the area. His next expedition (1863) took him to Madagascar and the Mascarene Islands farther east. Venturing into Somalia (1865), he sailed up the Juba River in the small steamship Welf, which foundered in the rapids above Bardera. There, he and three other Europeans were murdered by the Somalis.