[Spaceships] Orion space drive: launch from underwater?

[Johnny1A.2]

Originally Posted by Agemegos

I'm hoping not to have to be specific about that. In a superscience setting it might have to be a couple of klicks deep for the greeblies' civilisation not to have been detected by "scanners". In any setting in which the sci fi is as hard as flan or harder the greeblies will have their civilisation in the photic zone of depths and will build their spaceship on top of an isolated seamount, but orbital sensors won't be able to casually scan the oceans and find the ship and launcher.

Don't be too sure about that. Even within fairly 'hard' parameters, sensors may theoretically become frighteningly effective, esp. if combined with the right sort of software. It's mostly theoretical, but proposals have been made for things like using satellites to observe the ocean surface for the tiny swells produced by the passage of large objects deep down, or to scan for faintly warmer water heated by machines. Other things have been suggested that are compatible with known physics, and that might just work if you had enough computing power to process the data.