Lakes of Titan
It was possible that there were seas of liquid methane on Titan and first suggested after reading Voyager 1 and Voyager 2 information that showed Titan to have a thick atmosphere, capable of supporting lakes, but exact evidence was not found until 1995 when pictures from the Hubble telescope and other telescopes had already shown some proof of liquid methane possibly in lakes or planet-wide oceans, similar to oceans on Earth.