Getting into the secret room is a fairly simple task. However, due to its location at the end of the track, you'll need some expert skills to overcome the obstacles leading to it.
When you've reached the final checkpoint, take the lower route, where the afterburner is located. Slowly drive into the afterburner, then hold back on the thumbstick and hit the brakes to slow yourself down once it has activated. If you run into the wall ahead of the ramp, try to correct yourself and land back on the ramp.
Once you've gotten rid of the afterburner, slowly drive off the ramp and try to land in the gap between the ramp and the wall in front of you. Drive forward and you'll notice that a large container will fall away, opening a path into the secret room. This is the room where the map is located - just be careful of the explosives located at the end of the pathway.
InvestigationEditThe missing piece of the map is the Piri Reis Map, shown below. It is the oldest surviving map to show the Americas. It bears a date of 919 in the Muslim calendar, corresponding to 1513 in the Western Calendar. The map is a portolan chart, a common form at this time. Instead of latitude and longitude grids, compass roses were placed at key points with azimuths radiating from them.