I have an old post from SFO on a similar theme to this one, but I make the rules on this blog and this post isn’t violating any of them. (If you want to know what the rules are, let me know and I shall try and come up with some.) When I jet descends from altitude, the fuel the remains in the wing tanks will be very cold. The warmer and moister air lower down can result in the formation of ice on the underside of the wings where the remaining fuel is pooled. This Delta A330 shows that, as expected, there is fuel at the inboard end of the wings but also, less expected by me at least, there is a tank further outboard that still has fuel in it. I like the fact that fuel is kept further out since it alleviates bending loads. You can see the shape of the access panels where the ice is forming.