Back in August 2013, Google launched a feature named in-depth articles in their search results after a couple of months of testing. Since then, it really only had one major change in terms of how it appeared in the search results.
Seventeen days ago, in-depth articles completely dropped out of the Google search results. During that time, we emailed Google several times for an explanation and have not heard back.
Today, the in-depth articles are back in the search results, leading us to believe that this was an unexpected bug or outage with this feature.
The Mozcast features tool tracked the loss of in-depth articles in the Google search results over time. As you can see, they disappeared on April 9 and returned today, April 26:
From time to time, Google does have random bugs with their search results, but this outage seemed unusually long.