For this programme the team are tackling two environmental hazards: fire and pollution. Fire is a major hazard in the dry Colorado environment and Mike and Jonathan’s challenge is to build a carbon dioxide fire extinguisher. The basis of the extinguisher is a simple reaction of baking soda plus acid, which fizzes up to create the gas they want. But to make the acid they need Mike has to go out and find fool's gold and then work out how to transform it. Jonathan's task is to make the extinguisher itself. He needs to find a way to keep the components separate until needed. It's a tricky problem, but Jonathan works out a clever and elegant solution. Unfortunately, the first test goes badly wrong leaving him with less than an hour to fix it in time for the final test when they have to extinguish a blazing fire. Will the pair pull it together, or will their challenge go up in smoke? Meanwhile, Hermione and Ellen have an even tougher task. They have to find a way to clean up the polluted water flowing out of an old mine. The mine water is acidic and contains high levels of toxic metals and this mine run-off flows into streams and rivers killing off aquatic life. Hermione's plan is to use a rock - limestone - to neutralise the acidity. She manages to find a local source of limestone - and even proves that it works in a small scale test. But making a full size system that can cope with a continuous flow of polluted water is another matter... Ellen's task is to deal with the toxic metals in the water, and she harnesses the power of bacteria from a surprising source - horse manure. She builds a bioreactor hoping that once the bacteria start working they'll take the metals out of the water. But getting the system up and running in 3 days proves a very tough challenge. Waiting for them at the end of the test is a highly sensitive panel of judges - a group of water fleas will determine if they've made the water pure enough...