English Translation
How brave of you to go alone into the primaeval forest!
乗り込む is very unnatural for 原始林. We usually say:
一人で原始林に足を踏み入れようとは（あしをふみいれようとは）、君はなんて勇敢（ゆうかん）なんだ！
Note: 勇ましい and 勇敢だ are interchangeable, and 勇敢だ sounds more natural to my native ears.
乗り込む is used when 1) going into a place by a vehicle, 2) getting on a vehicle, or 3) going into “an enemy’s territory” with no fear.
原始林へ乗り込む cannot be interpreted as any of the three definitions.