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The most interesting discovery on Triton's surface was the presence of geyser-like volcanic vents (the black spots) that were apparently spewing nitrogen gas along with fine, dark particles. The Triton dust rose up to five miles high before descending back to the moon's surface. Triton also has a very thin atmosphere.
Voyager 2 revealed storms raging in Neptune's atmosphere. The largest one was named the "Great Dark Spot" which is a storm that's similar to Jupiter's Great Red Spot--including its geographic location on the planet. It has winds up to 1,000 miles an hour.
Astronomers could not locate this storm with the Hubble telescope after it was re-calibrated in 1994.
Voyagers continue to send back data, even after 30 years. The Cosmic Ray Subsystem team studies measurements of cosmic rays left. The data is sensitive to low-energy phenomena in interplanetary space including solar flares. This report is updated Monday-Friday before noon.
Here's a page with links to the most data (top left) returned by the two Voyagers.