The waters around Zanzibar are teeming with life and it’s surprisingly noisy.

This week it’s time for the team to investigate this underwater world. Their challenge is to find a way of eavesdropping on the sounds of the reef and also to have a look at what’s making all the noise. But the island is also home to some less desirable wildlife including the pesky mosquitoes so they also have to find a way of making a successful repellent. It’ll be Kate’s job to test it out.

Mike and Jonathan are given the task of making a hydrophone, or underwater microphone that can pick up the sounds of the fish down on the reef. Kathy’s job is to come up with a contraption that will enable the team to spy on the world beneath the waves. Meanwhile Ellen is in charge of keeping them all a mosquito-free zone.

Mike and Jonathan come up with an ingenious design for their microphone which involves them growing a very special crystal. Meanwhile Mike’s chemistry skills are also in demand from Kathy who decides to make a periscope. She needs Mike’s chemical wizardry to help her turn some glass into a mirror. It’s a difficult process with one very smelly raw material. And Ellen decides that the only way to prove her repellent really works is to raise her own mosquitoes, from larvae, and test their bites on Kate.