Elephant Seals (also called sea elephants) are large, ocean-going earless seals.
Before Ice StationEdit
A group of southern elephant seals took up residence in an underground cavern on the coast of Antarctica, which was formed when an earthquake destroyed a secret ice station building a prototype stealth fighter. However due to the plutonium core powering the plane, the leaching, passive radiation gradually began to affect the seals, causing them to grow larger and become more aggressive at night.
Presumably they were all killed by the nuclear explosion that destroyed Wilkes.
The mutant elephant seals are diurnal, so they were only aggressive to the diving teams that went down at night, they were also larger than regular Elephant Seals and the males were known to have larger tusks than normal, large enough, in fact, to kill a Killer Whale.