Long thought to be only the imaginings of maintenance workers, these creatures have only recently been confirmed as real. The beasts spend most of their time below the surface of water/sewer sludge, only surfacing to scout the area briefly or to attack. Dianogas have been known to sieze their victims with tentacles and drag them underneath the water to drown, but it is most common that they attack with their massive jaws.
Suggested Method of Attack - You won't see a Dianoga at first, but you can hear a guttural growl when they notice your prescence. Look for the eye-stalk to poke up above the sludge and try to blast it as fast as you can. Explosives are only good if the creatures are near a wall because TDs and mortars will only splash in the water. Your best hope is to find a ledge above the water/sludge and pick off as many as you can before jumping in. When in doubt, run; these things aren't too fast.