Thin crack in granite behind the sink is leaking water

tl;dr What is causing (and what should I do) about a hairline crack in my granite countertop that is about a foot long but goes all the way through and causes water to leak into the underneath cabinet?

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I noticed water leaking into the cabinet under my kitchen sink. It's hard to see, but eventually I found that the water was leaking from a very thin crack in the countertop granite between the sink and the backsplash. The crack is just over a foot long and less than a mm wide with no height separation. When I looked at it from underneath the counter, the crack underneath seems worse. I've attached 2 pictures of the crack from above, and two from underneath the counter (both with a wide look and close-up of the crack).

My house is only 12 years old, and I thought granite was supposed to last a lifetime. Aesthetically I don't mind the crack, but I'm worried about it worsening, and the leaking definitely needs to be fixed. What is going on here? I'd love some advice before I either put some think caulk into it to stop the leaking or hire a granite repair person (which I assume will be very expensive).