of paths in different colours and volunteers use it to check what needs surveying. purple shows new paths yet to be audited, like the one we're standing on, and then the green shows a completed path. just looking at the map, you can see the majority of the 40,000 miles of paths still need to be audited. but if more volunteers come out and enjoy notjust the walks but the technology too, the team will only get closer to achieving their aim. spencer: now, earlier in the programme, we'd met back up with gav, from the slow mo guys, to film a rocket engine test at 80 times slow motion. that's 2,000 frames per second. and here's how we got on. we're in a nice, flat part of texas. seems a sensible place to do a rocket test, doesn't it? and as you walk along, you start to get hints that, yeah, there's something going on here. lots of compressed nitrogen in canisters. then you get to this shed.