Question: What is a sitemap of sitemaps and how can it be implemented?

"Sitemap of Sitemaps", also known as a Sitemap index, is a way to organize multiple sitemaps into one place. This is especially useful for larger websites that may have millions of pages and therefore more than one sitemap file.

Google supports two types of Sitemap indexes: simple and complex. A simple Sitemap index contains references to other Sitemap files. A complex Sitemap index can contain references to other Sitemap indexes.

Here's an example of a simple Sitemap index:

<?xml version="1.0" encoding="UTF-8"?>
<sitemapindex xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
   <sitemap>
      <loc>http://www.example.com/sitemap1.xml.gz</loc>
      <lastmod>2004-10-01T18:23:17+00:00</lastmod>
   </sitemap>
   <sitemap>
      <loc>http://www.example.com/sitemap2.xml.gz</loc>
      <lastmod>2005-01-01</lastmod>
   </sitemap>
</sitemapindex>

Each <sitemap> tag defines a separate sitemap with its location (<loc>) and the last modified date (<lastmod>).

Please note: each individual sitemap that your Sitemap index includes must not exceed 50MB (uncompressed) and they must list no more than 50,000 URLs. If you're exceeding these limits, you need to create additional sitemaps or use a Sitemap index. Also note that a Sitemap index file itself must not exceed 50MB (uncompressed) and can include up to 50,000 sitemaps.

By using a Sitemap index, you can effectively communicate with search engines about the content they should crawl, improving your site's visibility and SEO.