It's Mine! All Mine!
It's a good thing Indy doesn't have achluophobia (fear of darkness) because this movie is dark. A big part of the movie is about confronting fears, like the fear of insects (entomophobia), fear of enclosed spaces (claustrophobia), or the fear of having your heart ripped out by a crazy cult leader (heartrippedoutophobia).
So it's fitting that Temple of Doom takes place deep underground. It wouldn't be very scary in broad daylight. And for about an hour of the movie, we don't get any sunlight at all. It's a relentless onslaught of gore, mind control, and child slavery.
But it can also be seen as a metaphor for our characters looking inside themselves. Willie, whose eyes sparkle at the thought of diamonds, realizes that some things are more important than jewels. At the end, she doesn't even suggest keeping the diamond-filled Sankara stone for herself…although she does question why Indy gives up his shot at fortune and glory.
And Indy realizes that being an archeologist isn't just about getting the treasure. He helps to save a culture. And maybe after witnessing all that child slavery, he'll reconsider keeping a young boy as his assistant.