This week the scientists are set their most ambitious challenge yet – to protect a coral reef. Chumbe is a tiny island off the coast of Zanzibar which is home to one of the most diverse reefs in the Indian Ocean.

Four hundred species of tropical fish and two hundred kinds of coral thrive there. But this unique underwater world is under threat, from fishing boats and passing ships. The team have the task of inventing some sort of system which can alert boats that they are entering a protected area.

It’s a group challenge and the team come up with an unusual solution – possibly the world’s first coral reef burglar alarm! Mike and Jonathan have the task of making an alarm system to warn the boats. Chumbe is an eco-island so any power they need has to come from a renewable source – batteries are out.

Meanwhile Ellen and Kathy have the job of anchoring the system in place. They know that the exclusion zone, which rings the reef, is 300m out from the shore but they have no clue how deep the water is there. In order to fix the alarm in one spot they’re going to have to find a way of measuring the water depth so they can accurately measure the anchor ropes, but with strong currents nothing is as straight forward as it sounds.