This is the 7303rd most frequent Japanese word.
乗り込ん
"乗り込ん" means "board" or "get on" (a vehicle, ship, etc.).
Here, '乗り込ん' is used to express physically boarding or entering a vehicle, in this case, a bus, indicating the start of their travel.
彼らはバスに乗り込んで旅行を始めました。
They boarded the bus and started their journey.
In this sentence, '乗り込ん' metaphorically addresses entry into a place, referring here to the transition of entering a cave for exploration.
探検家たちは洞窟に乗り込んで調査を開始しました。
The explorers entered the cave and began their investigation.
This sentence uses '乗り込ん' in a more figurative sense, representing boldly entering or taking part in challenging circumstances, here implying entering enemy territory.