On an individual page, search engines need to see content in order to list pages in their massive keyword–based indices. They also need to have access to a crawlable link structure—a structure that lets spiders browse the pathways of a website—in order to find all of the pages on a website. Hundreds of thousands of sites make the critical mistake of hiding or burying their main link navigation in ways that search engines cannot access. This hinders their ability to get pages listed in the search engines' indices. Below is an illustration of how this problem can happen: