Here's the problem: A leaky faucet. Somewhere up in the guts of the faucet mechanism, a seal has failed, and water dribbles into the cabinet below every time someone uses the sink.
Things could have been a lot worse... The faucet could have had a constant slow leak, turning the wall to mush and filling the cabinet with mold before anyone noticed. Water damage is evil.
Aside from a replacement faucet, I needed two new things for this operation:
That's a "telescoping basin wrench" and a set of "pipe-wrench pliers" with little silicone covers so they can work on fragile things.
The replacement operation took about four hours, including the research I did online and the shopping trip I made on my bike. The trickiest part was getting the basin wrench up behind the sink so I could unscrew the old hoses. If I hadn't learned about that tool from an online video, the operation would have been nearly impossible.
A nice little bit of amateur-level maintenance, and a few more tools for the basement.