Bed drains

Sorry if this is a stupid question but I noticed some standing water in my bed this morning. The driver side drain hole was clogged. I took a small screw driver to clear it out, and while I was able to clear out the gunk enough for it to drain, the tip of the 4" screwdriver was hitting something beneath the bed. The center and passenger holes had no obstructions. I can't tell if I was poking at something important, or if it was just a lot of crud in there. I didn't jam it real hard, so if something important is there, I hope I haven't damaged it. Anyone know what is under that hole?

 

My guess is gas tank.