What causes extreme, exessive thirst ? i've been drinking water all day. but nothing's quenches my thirst.

Many possibilities: Causes of extreme thirst include recently eating highly spiced or salty foods; extreme dehydration from sweating, etc; bleeding enough to significantly decrease blood volume; diabetes and diabetes insipidus; certain medications; severe infections; heart, liver, or kidney failure; and psychogenic polydipsia. See your family physician for a checkup, preferably today.