When people figured out that the Google crawler was scanning the page for keywords, so it could understand what was on there, they soon realised that if you filled your page with popular keywords, you would soon appear in all sorts of searches. So in the late 90s there were a glut of pages that mentioned Britney Spears for no reason than to get found.
When people figured out that PageRank was basically counting the number of links going to your site, they built their own "bots" that just posted links to your site anywhere and everywhere. Google spotted these thousands of links, took them as a sign that your site was truly popular, and pushed you to the top of the rankings.
This carried on for years; Google tweaked the algorithm, changed how it analysed pages, amended how it ranked pages, but the SEOs just responded and adjusted their tactics to match.
It was an arms race, and we, the people who were looking for things, were the losers.
Take action: How did Google get around this? Answers on a postcard please