The Gallifreybase forum is going nuts right now. Three pages of posts in the past couple of hours. This is the best find in nearly 20 years.
Two clips have been posted on the BBC site, both are marvelous. Although neither of the stories were high on most people's lists, the particular episode of Underwater Menace is lucky because it contains some visuals that many have been curious about (audio of every episode exists - it's video that is missing). Some are already speculating that the quality of Underwater Menace may be higher than people had assumed.