Question

what is the secret of the lake in palau?
there are millions of golden jellyfish
it contains ancient fossils
it has glowing coral reefs
its home to box jellies

Explanation:

The lake in Palau is famous for having millions of golden jellyfish. This unique ecological feature is what makes it notable. Ancient fossils are not a key feature of this lake. Glowing coral reefs are more associated with some oceanic coral reef areas rather than this specific lake. Box jellies are not the main secret of this lake compared to the golden jellyfish.

Answer:

There are millions of golden jellyfish