I downloaded the photo and turned it upside down for a better view. It didn't help much.

What year and model is your car? That info would help greatly. The hole you're pointing to looks like a trunk drain. Normally, water could drain out of there, but diesel shouldn't.

For no reason, I'm assuming this is a W123 sedan. I would guess that your diesel tank sprung a leak inside the trunk. The best next step would be to remove the in-trunk access panel to see if you can determine the source of the leak.