Anna Du was walking along Castle Island’s beach in South Boston when she noticed plastic scattered on the shoreline. She reached down to pick it up, and quickly realized there was many more tiny pieces than she could handle.

“When I realized how many pieces there were, it seemed impossible,” says Du, who was in sixth grade at the time.

But Du approached the problem like any good scientist—first, by doing a little research. That’s how she learned that 8 million metric tons of plastic end up in the oceans every year—and that’s in addition to the whopping 150 million metric tons that are already there.

Then she got to work building something that could help solve the issue: a remote-operated vehicle, or ROV, that can move through water and spot plastics on the ocean floor.

Du is one of 30 finalists in the Broadcom Masters competition, one of the top science, technology, engineering and math (STEM) contests for young students in the world. The competition, organized by the Broadcom Foundation and Society for Science and the Public, has been running for eight years. This year, nearly 5,000 sixth, seventh and eighth graders were nominated after competing in regional science fairs, and of those, 2,500 applied online for consideration. That pool was narrowed down to 300 projects dubbed “Broadcom Masters,” and now the top 30 finalists are currently gathered in Washington, D.C. to showcase their projects. A top prize of $25,000, as well as other cash prizes, will be announced today at the Carnegie Institution for Science.

Du’s ROV is made with PVC pipes. Inspired by ROVs of all sorts, like the Curiosity lander that spies on the surface of Mars and the deep sea arctic Nereid ROV at Woods Hole Oceanographic Institution (WHOI), Du’s ROV has two separate systems, a navigation system and a detection system. The navigation system is pretty simple: it uses propellers to move through the water and a novel combination of fishing weights and foam pool floats that allow it to move up and down.

“The real invention here is the sensing,” says roboticist and engineer Dana Yoerger, who works with WHOI’s Nereid, a ROV that travels great distances in the arctic and hosts a suite of acoustic, chemical and biological sensors on board. In awe of Nereid, Du nicknamed her ROV Nereid Jr., even though her vehicle has a different skill set than its namesake. “The ROV is nicely done for a 12-year-old and hers is quite clever,” Yoerger adds.

The detection system, Du agrees, is the “actually cool part.” She uses a high resolution infrared camera along with three different kinds of light to spot the plastic. Du’s detection methods are composed of two different wavelengths of infrared light, which illuminate the absorption spectra in microplastics and make them stand out from the sand and plant life. She also uses visible light to spot unnatural colors that might make the plastics stand out. (Du was also one of ten finalists in the Discovery Education 3M Young Scientist Challenge this year. You can see a demo in her application video.) The apparatus doesn’t actually collect the microplastics, but it does identify where they are accumulating. She has applied for a patent on the functional elements of the ROV.

“She has an impressive basic kind of engineering instinct to break down a problem like this and then go after it,” says Casey Machado, who is also a WHOI engineer who works with Nereid. “She was able to follow that up with the technical work, construction and design to make a working prototype, which is very cool. It’s sounds simple, but it’s a level of thinking that’s really impressive.”

When Du first read about the challenge of locating ocean microplastics that, unlike the Great Pacific Garbage Patch, are not floating on the surface, she knew that an ROV would be the most efficient tool for the task. Taking buckets and buckets of water samples and analyzing them in a lab could never be done to the scale needed. She needed a mobile lab that could find the plastics in-situ.

Du has been attending public events and workshops at MIT since she was five years old, and so she picked up the engineering skills necessary to build her ROV from these sessions, maker labs at local libraries and YouTube. She says actually getting her device to move through water well was tricky—even down to choosing the right kind of glue to use to hold the PVC pipes together. Before she added fishing weights, for example, the ROV would flip over when she tried to move it up and down. There was a lot of trial and error as she tested her ROV in Boston Harbor.

“Anna is a force to be reckoned with—all of these finalists are,” says Maya Ajmera, the President and CEO of Society for Science and the Public. “What’s amazing to me about all of the kids that go through our competitions is they look at big problems. They look at a big problem and narrow it down until they get to something feasible that they can do something about. With Anna’s project, you can see the big picture.”

That’s Du in a nutshell for sure. When asked about future plans, she mentions wanting to somehow address the effects of climate change.

“Especially with climate change happening all around the world, I think there’s a lot of problems that could be solved with new inventions,” says Du. “Right now, I’m mainly just focusing on plastics because there’s still a long way to go.”

For Nereid Jr., Du’s next step is developing an artificially intelligent system that can create a predictive algorithm of where microplastics are likely to aggregate.

Du credits her parents, who for years have been taking her to MIT’s student outreach activities on weekends, for fostering and supporting her interest in STEM. She says she has been able to meet students and scientists there, and, as a result, has her sights set on attending the university and becoming an engineer.

“I know I want to be an engineer because I like building things to help solve world problems,” says Du. “But I’m not sure what kind of engineer I want to be yet.”

At her age, she has plenty of time to figure that out.