Storyline and can not be missed.
You will earn this once you defeat all 3 guardian siphons in the Autopia stage and cave appears with the new project.
*** Spoiler - click to reveal *** I found the best way to defeat them all was the pressure plates to avoid the puzzle element.
Thought I would add a little help with the tunnels between Siphons.
Tunnel 1 is simple you and oswald act together. Tunnel 2 literally "wait" when gus tells you too, but make sure to keep hold of the switch.