Rafting is accepted theory for the spread of many species, New World Monkeys for example. Of course in a fantasy realm you take any natural event and turn the volume up to 11. So you get elephants rafting, of course why not make it a huge fantasy animal?
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Why raft? Without natural predators, elephant populations can be very destructive to the ecosystem, a population which thins its number from time may actually prove to be more stable over the long term. Thus the rafting herds would over take the boom crash herds over many years. But you could make up other reasons...
A turbulance and heat coming from this sacred site has scared the locals. Those that have dared to dive and investigate have not returned. Those brave enough to visit it at night have seen an increased turmoil and a changing of colors reflected in the steam emitted from amongst the ancient columns.
--A latent spell reaction, harmless except for the heat and uncontrolled magic in the immediate vacinity.
--The magical bind that trapped the beast is weakening and a magical war has erupted to gain its freedom once again.
--A local cult has found a key to the writings underneathe and and trying to convert its use for their own cause. The location and magic inherent in the creation are there if only its power can be tapped.
--Recent storms have dislodged the perfectly positioned columns. If they are not corrected then the magical prison will soon release what it contains.
--It is a constant prison continuing to pull in and trap other creatures of similar style of what it was originally created for. Like the Ghostbuster's containment vault, the quantity of creatures is reaching critical mass. Soon the locks will fail releasing a unimagineable amount of wreckless evils into the surrounding country. Those swimming down can see the writhing masses pushing against the prison walls. Those fools who went down earlier were either captured for going to close or died in fear when they gulped down water upon seeing the horrors.