Humans love to eat fish, but we must be careful not to catch too many. To make the right rules about how many fish can be caught without decreasing the population too much, it is helpful to know how many fish are in the sea. It is difficult for scientists to go underwater to count fish, but technology can help. Animals like dolphins can use sound to “see” the world around them. Just like dolphins, scientists can send a sound into the ocean and measure the echo that comes back. They can even use the unique echoes of fish to get an idea of how many fish are in the sea. Scientists are now testing whether this technology can help them to explore the deep sea. While it is not always easy to see with sound, the challenges we face and the mistakes we make often lead to new discoveries!