This guide belongs to Dulfy. All credit to her, you can visit her website for the original guide: dulfy.net
GW2 Tangled Depths Mastery Insights and Strongboxes Guide.
There are a total of 6 Mastery Insights and 5 Strongboxes, granting you a total of 11 Mastery points and 16 AP. One of the strongboxes has not yet being discovered and one of the Mastery Insights could be bugged currently since it is granted at a different location.