Ok, well, I'm fairly obsessed w/ the idea of the passage of time and what I think of as "circles" in life. Like when we find ourselves somewhere we once were before, by accident or unexpectedly, or even places we return to over and over again throughout different points in our lives. For example, my family goes to the same spot in Florida every summer. Same town, same condo building, and for many years the exact same condo (though that has changed several times). This goes back to when I was like 10 years old. We've changed dramatically in the last 25 years, but these places stay the same (more or less). So each time we end up there, that's a circle completing.
Or when I was in high school and I'd drive to Atlanta once or twice a month to play in my punk band, and the route I took always took me by a street called Curran Street (I noticed it cause I have a friend whose last name is Curran). 8 years later or so I ended up living on Curran street. That's a circle.
I could list a hundred examples, most minor. But the point is I find this fascinating how we change and move on while these places continue to exist without us, then we find ourselves there again.
So with this in mind, consider something like the pyramids, which have stood there for thousands of years. Millions of people have come and gone, and they remain, more or less the same, day after day. Pushing this idea to its extreme, I started thinking about a future scenario where the earth was flooded, and these structures were still there, oblivious to the world changing around them. Now, instead of pharaohs and slaves, or tourists and tour guides, the pyramids are surrounded by sea creatures that know nothing at all of the vast history of the pyramids themselves.
Essentially what I did was think about this theme of time passing and worked my way around to imagery that I like a lot on its own (the sea and pyramids). Over the past year or so the idea has gone through different variations w/ different images (hourglasses, skulls, crashing waves, etc) before I arrived at the simple explanation of "ancient Egypt + sea creatures", which is what I told the artist initially. Turns out he's into a lot of the same kind of ancient alien/OOP artifact stuff that I also like, so he pulled it all together wonderfully.