Yes, it’s from a book called “Uncle John’s Bathroom Reader” and that book was resting on a closed toilet seat in a nation that only kind of uses the metric system in certain situations… however, you’d be surprised how much you can learn after a single visit to any local fast-food joint!
Megameter, gigameter, and terameter are occasionally used in astronomy and Earth science, though units like AU and light-year are usually preferred.
Here’s an example of megameter being used. https://www.science.org/doi/10.1126/science.abd0312