The Fel Pits are a series of fetid and scalding pools lining the western base of the Hand of Gul'dan in central Shadowmoon Valley. Tainted and enraged earth elementals and fire elementals roam the area.
This area is a well known grinding spot for Mote of Fire, and can often be crowded at prime time. Furthermore, classes with pets and melee classes should always pull mobs inside the lava out of it before attacking, even if you have to receive damage from a spell (like Fel Firebolt from fire elementals), for lava deals loads of damage if you stand in it. Lots of fire elemental roam around, in fairly close proximity, but there are also earth elementals that drop motes of earth that are much less valuable on most servers.