He should build a sub to go to the bottom of the ocean.

Ah yes.

It depends on what you are into. If you like anime (for example) you could try something broad like # (anime) [no spaces and such] & then see what other hashtags people are using from there. The same with music or something. That’s what I do at least. There might be better ways but a lot of times people are into similar stuff and that can lead down different paths as well.