Neptune's largest moon, Triton, is one of two satellites in the Solar System that are currently geologically active. At least two geyser-like plumes were observed by the Voyager 2 spacecraft in 1989, and dozens of streaky deposits hint at the existence of many more. Triton also exhibits complex seasonal changes in its 165-year journey about the Sun. Because Triton's atmosphere transports volatiles (primarily nitrogen and methane) during this seasonal cycle, its atmospheric pressure may fluctuate by up to an order of magnitude over decades. Photometric measurements of its albedo and colour over half a century show that seasonal volatile transport has occurred. There have also been indications that more extreme, short-lived changes, perhaps due to geological events, have occurred on Triton. An anomalously red spectrum was reported for Triton in 1977 (refs 5, 6), and global warming has now been observed.