The in-depth articles feature launched by Google in August 2013 has received an update. The new additions were announced on Google+.
Google has added a link "More in-depth articles", under the main block of in-depth articles. When you will click on the link, you will be shown 10 additional articles. Besides this, Google has also added the explore section next to an article that will let you explore related topics with a keyword search.
Here's a screenshot:Google Expands the In-depth Articles Feature!,