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.
He took on the dangerous mission with the resolve to jump into the lion's den.
From Chinese 虎穴 (hǔxué), literally 'tiger's den'. The figurative meaning is shared across Chinese and Japanese.