This issue of Oceanography focuses on seamounts, submerged isolated mountains in the sea that are hosts to complex and intertwined, dynamic physical, chemical, and biological systems. More than 99% of all seamounts on Earth remain unexplored, but studies of a very small subset so far show that they have and continue to provide exciting research opportunities across many science disciplines. Magmatic processes that originate deep in Earth’s mantle build volcanoes—seamounts—that can reach heights of up to 9 km above the seafloor. Volcanically active seamounts develop hydrothermal systems that vent hot, metal-rich, reducing, and often acidic fluids and that result in a considerable chemical exchange of elements and volatiles with seawater. In addition, many seamounts provide major fluid pathways between the underlying oceanic crust and the ocean, bypassing the impermeable sediment layer covering most of the oceanic crust. Seamounts also interfere with ocean currents to yield local circulation patterns that help mix the ocean, resuspend sediments, and perhaps locally trap or enhance the abundance of plankton.