The waters around Zanzibar are teeming with life and they are surprisingly noisy. The team’s challenge this week is to eavesdrop on the underwater sounds of the reef to discover what is making all the noise. Jonathan and Mike will work together to construct a hydrophone – an underwater listening device. Kathy has to build a contraption that will allow them to see under the water. Meanwhile, Ellen has to fend off some of the island’s less desirable wildlife – mosquitoes.

• Kate Humble
• Iain Stewart