The lake is cut off from the main ocean, which means the jellyfish inside it have been living separately for a very long time. With no predators hunting them, they no longer needed their sting to protect themselves, so it gradually became so mild that humans cannot feel it at all. They are perfectly safe to swim alongside.
The golden jellyfish follow the sun because tiny plants called algae live inside their bodies. Those algae use sunlight to make food — and the jellyfish get some of that food too. So the jellyfish swim towards the light all morning, then turn around and swim back in the afternoon. They do this every single day like clockwork.
Snorkellers who visit Jellyfish Lake describe it as one of the most magical experiences in the world — floating in warm, still water surrounded by millions of softly pulsing golden blobs. The lake also has a second species, the moon jellyfish, which is translucent white and almost see-through.