tiger's den; lion's den; very dangerous place
Figurative expression for an extremely dangerous place or situation. Often used in the proverb 虎穴に入らずんば虎子を得ず (Nothing ventured, nothing gained).
Going into the enemy's stronghold is like entering a tiger's den.
彼は虎穴に飛び込む覚悟で、危険な任務を引き受けた。
He took on the dangerous mission with the resolve to jump into the lion's den.