It's another cave, all right. Could be as big as the house, judging from the number of bats that came out of it.
9 seconds sound clip from the Batman: Mask of the Phantasm movie soundboard.
You can hear this line at 00:34:52.684 in the Blu-ray version of the movie.
Quote context
[...]
- Uh-oh! Looks like dad's got company, business-type company.
- He doesn't usually see clients here, at least not at this hour.
- Maybe we should wait till tomorrow before giving him the good news.
- Maybe.
- Miss.
- Left with dad... Too young... Need time... Forget about me.
- My God!
- If there was just some other way.
[...]