A swimmer in Wales filmed a giant jellyfish in the sea. He first saw a few long arms called tentacles in the water. Then he saw more and more tentacles coming. He said the animal looked as long as a bus or a small train.
The sea animal was a lion's mane jellyfish. It is one of the longest animals on Earth. The body of this jellyfish can grow up to three meters long. Its tentacles can grow to more than 30 meters long.
Experts say the tentacles are very thin and hard to see in the water. The tentacles can sting people even after they break off from the jellyfish. The sting is very painful, but it is not dangerous to humans.