-( Story related, cannot be missed )-
Step one: go to the gorge and read everything there is to read on the plaque.
Step two: proceed to the Mine Road.
Step three*: acquire a plank (while other weapons will soon fill your arsenal, the plank is all you need for this).
* You will need to also meet and acquire another booster off of one of the other bikers, but as you're trying to find your targets, you might acquire this sooner than later.
Step four: with the plank in hand, encounter a Cavefish (noted for their previous cutscene involvement in destroying the gorge bridge).
While fighting a Cavefish, make sure you're not too close to it or it will squirt oil on the road and immediately take you down. Instead, wait for it to stick its head up, then wallop it with the plank to knock it out and give you the goggles you need. If you use any other weapon, it will self-destruct the bike after you knock it down.
Step five: with goggles in hand, equip them and watch the rail-ran scenario as you scope out the cave. You will see that it will flash Exit as you get back to the highway ramps, but what you really want to watch for is when, instead of Exit, it shows Cave. Press X then to move along with the story.