B2 noun /den/
Den
巣穴; 快適な部屋; 不法活動の場所
Meaning
a wild animal's lair or home; a small, comfortable room; a place for illegal activities
Example
The fox disappeared into its den when it sensed danger approaching.
キツネは危険が迫るのを感じて、巣穴に姿を消しました。