Reaper Leviathan remains can be found on the floor of the Inactive Lava Zone, and within the Mountains corridor. They are either found as whole skeletons, or just skulls. The Reapers came to be in these locations due to Sea Dragon Leviathans making infrequent trips to the surface to hunt them, and then bring them back down to eat due to their natural food sources being decimated by Kharaa.
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Reapers are ill-suited to high temperature environments, suggesting this creature was forced down into the active lava zone by conditions outside its control.