Sonar's done a lot for us already, but it's still being developed for different technologies like

• satellite sonars.
• synthetic aperture sonars.
• sound cloaking devices.
• dolphin-inspired sonars.

They all sound like technology from arm-waving, vague, early 20^th Century sci-fi depictions of the year 2000, but…they're real, and they're actually being developed.

Synthetic Sonar

Synthetic aperture sonars use a couple of different antennas to send out pings, using the overlap to get a better picture of the object. They also work at lower frequencies, which travel much deeper than standard sonar. The only problem is that you can't get results immediately when you're using this kind of sonar. Instead, the sonar collects data, which you have to give to a computer to process. Once the computer processes all that data, though, you've got a much better image of what's going on in the object. Given the rate of absorption from an object, you can tell a lot about its shape.

(Source)

Cloaking Devices

Invisibility might be nearly impossible for anyone who doesn't have an invisibility cloak lying around, but sound might be something we can actively control. Sound has an absorption rate that differs depending on the material, which could mean that some materials can just stop sound in its tracks.

Materials like rocks and metals are really good at absorbing sounds because they're incompressible (the sound wave has a hard time compressing them). Right now we're working on creating or finding more materials that are really good at making sound disappear.

Dolphin-Inspired Design

We may have our own, human-created sonars, but dolphins have thousands of years of evolution on our tens of years of engineering. Right now we're researching the ways that dolphins make sonar work to make improvements on our own designs. We'll never have it built in the way dolphins do, but understanding how they make sonar work can inspire our own mechanical versions.

While the future for sonar might seem limitless, there's still a key limitation we need to work around. Because of the way sound waves work, there's a limit, called the diffraction limit, that tells us the resolution of a sonar image can never be more than half of the displacement of the waves (the amount the waves displace water molecules).

All that matters because it limits just how clear of an image you can create using sonar waves. We create sonar images from back-projection, but with ambient noise and the diffraction limit, most sonar images have a poor resolution.

Hey, at least they won't give us cancer.