Across centuries, a handful of rulers, tycoons, and modern tech founders have amassed fortunes so vast that, once translated into today's dollars, they eclipse almost anything on today's rich lists.

America has more billionaires than ever, led by Elon Musk, who's estimated to be worth $744.6 billion. Figures such as John D. Rockefeller and Andrew Carnegie were known as the richest men of their ...