Junk in my fuel?

[MUDDVOL]

I am replacing the fuel pump and also the fuel bowl heater. AFter taking out the filter and heating element it looks like a I have sand and some metallic junk in the bottom of the canister. Where did that come from?
Do fuel lines rust and some of it get sucked up in the filter housing or is my fuel tank rusting on the inside, the rest of the bottom of the truck sure is? Anybody got any ideas about this kind of junk or other explanations?

[ryandamo]

diesel fuel is pretty dirty compared to gasoline, alot of it probably just accumulated over the years, also someone may have gone to long without draining the thing, if you get a q-tip and scrub around at the bottom then pour some more diesel or whatever in there with the drain open it should help clean it out some