The hiring process at today's biggest tech companies can get pretty competitive. Some of the infamous, unanswerable questions asked at Google job interviews include “How much should you charge to wash all the windows in Seattle?” and “Why are manhole covers round?” Such questions have since been banned from the company’s hiring process, but back when they were allowed, Google was doing something even crazier to weed out job candidates.

In 2004, Google paid for billboards in Silicon Valley and Harvard Square that displayed a web address in the form of a math puzzle. The message read "{first 10-digit prime found in consecutive digits of e}.com”, with no sign of the company’s name on the ad. For onlookers who were tenacious enough to solve it, the answer (7427466391.com, of course) brought them to a website that presented an additional problem. It was only after cracking the second puzzle that the web page revealed it was all part of an unorthodox recruiting stunt from Google. From there, math whizzes were able to submit their resumes. The final web page read:

"One thing we learned while building Google is that it's easier to find what you're looking for if it comes looking for you. What we're looking for are the best engineers in the world. And here you are. As you can imagine, we get many, many resumes every day, so we developed this little process to increase the signal-to-noise ratio.”

Google receives more than 3 million applications each year, and only 0.2 percent of them lead to actual hires. Though it makes sense for the tech giant to want to narrow down the resume pool, they haven’t forced their applicants to complete scavenger hunt–esque puzzles since 2004. Today, Google says they make their hires based on qualities like leadership, cognitive ability, and “Googleyness.”

[h/t: NPR]