I can't see them

Jimmy lived in the country, and he loved playing in a very shallow river near his house; but then his father got a job in a big city, and he moved there with his family.

Their new house had a garden, but the garden was very small. Jimmy wasn't very happy. 

"Is there a river near here?" he asked his mother on the first morning.

His mother answered, "No, there isn't, but there is a beautiful park near here, Jimmy, and there is a pool in it. We'll go there this afternoon." Then Jimmy was happy. 

After lunch, Jimmy and his mother went to the park. Jimmy wanted to walk near the pool, but there was a sign in front of it. His mother read it to him: "WARNING: This pool is dangerous. 367 people have fallen into it." Jimmy looked into the pool carefully. Then he said, "I can't see them."