Google had updated the in-depth articles feature they launched back in August 2013.
The new additions, as they announced on Google+, adds more ways to see more in-depth articles within the search results.
Google added a link under the main block of in-depth articles to “More in-depth articles.” Clicking on that link will show an additional 10 articles. Google also added the ability explore related topics by adding an explore section next to an article, with a new keyword search that would show more in-depth articles on the related topic.
Here is a screen shot:
If you want to just search for in-depth articles, Ruben noted that you can add &ida_m=1 to the end of your search URL.