The phrase "dense underbrush" refers to an area of thick and closely packed vegetation that grows low to the ground, such as bushes, small trees, or tangled plants. Full definition
The ticket office has been cleaned out and prettied up since Koch's time, when balls were sometimes lost in dense underbrush. (si.com)
We have been scratching our way through dense underbrush in northwestern Australia, guided only by the dim light from a GPS screen. (scientificamerican.com)
During the hike, try to aim for soft terrain and stay clear of dense underbrush and sharp rocks since these can damage paws. (totallygoldens.com)