Subterranean hideout in comics

• Robin's place
• Secret DC headquarters
• Superhero's lair below the Wayne Manor
• Hideout for masked men
• Robin's milieu
• Grotto under Wayne Manor, and an apt title for this puzzle
• Comic-book hero's hideaway
• Lower part of Wayne Manor
• It lies beneath Wayne Manor

Wikipedia Word definitions in Wikipedia
The Batcave is the secret headquarters , command center and safe house of the DC Comics superhero Batman , the alternate identity of playboy Bruce Wayne , consisting of a series of subterranean caves beneath his residence, Wayne Manor .