A 14-meter-high extinct hydrothermal vent chimney was discovered on Dive 7 of the 2016 Deepwater Exploration of the Marianas expedition at Fina Nagu Caldera A on top of a volcanic dome. This hydrothermal chimney was probably once a high-temperature black smoker, built by hot, chemical-rich fluids created by seawater percolating downward and interacting with hot subsurface rocks above the dome’s magma chamber.