Ernst Haeckel (1834–1919) was a German-born biologist, naturalist, evolutionist, artist, philosopher, and doctor, who spent his life researching flora and fauna from the highest mountaintops to deepest ocean. A vociferous supporter and developer of Darwin’s theories of evolution, he denounced religious dogma, abandoned an early career in medicine, authored philosophical treatises, gained a doctorate in zoology, and coined scientific terms which have passed into common usage, including ecology, phylum, and stem cell. Haeckel’s colossal legacy has fascinated, confounded, and polarized generations. But what was at the heart of his extraordinary life’s work? Rather like his intellectual forebear, Alexander von Humboldt, Haeckel was motivated not only to discover but also to explain. To do this, he created hundreds of detailed drawings, watercolors, and sketches of his findings which he published in successive volumes during the 20th century, including several marine organism collections and the majestic Kunstformen der Natur (Art Forms in Nature), which could serve as the cornerstone of Haeckel’s entire life project. Like a meticulous visual encyclopedia of living things, Haeckel’s work was as remarkable for its graphic precision and meticulous shading as for its understanding of organic evolution and cellular development. From bats to the box jellyfish, lizards to lichen, and spider legs to sea anemones, he emphasized the essential symmetries and order of nature, and found biological beauty in even the most unlikely of creatures. The prints not only furthered the study of natural history but also influenced generations of 20th-century artists and architects, from the emerging proponents of Art Nouveau to architects such as Hendrik Petrus Berlage, whose Amsterdam Commodities Exchange was inspired by Haeckel’s illustrations. In this book, we celebrate the scientific, artistic, and environmental importance of Haeckel’s work, with a collection of prints from several of his most important tomes on marine biology, including Die Radiolarien, Monographie der Medusen, Die Kalkschwämme: eine Monographie, and Kunstformen der Natur. At a time when biodiversity is increasingly threatened by human activities, the book is at once a visual masterwork, an underwater exploration, and a vivid reminder of the precious variety of life.