"Dense undergrowth" refers to a thick and closely packed growth of plants, usually found on the ground or at a lower level in a forest or jungle. Full definition
He avoids such details especially when, forming» [a] dense undergrowth of mutually contradictory hypotheses» (p. 104), they threaten to impede an encounter with Jesus. (faith.org.uk)
In parts, the trip required our best Indiana Jones qualities as we hacked our way through dense undergrowth to uncover uncharted Mayan burial sites visited alas only by grave robbers. (totalpolitics.com)
Mother cats usually hide their kittens very well and choose places such as in or under garden sheds, in dense undergrowth, and in or under discarded rubbish such as old furniture. (celiahammond.org)