This week in the Planet Earth Podcast, how last year's eruption of the Eyjafjallajökull volcano in Iceland gave scientists an unparalleled opportunity for research, and why sediment from rivers like the Thames can act like time machines to bygone eras. The volcano disrupted millions of people's journeys, but as the saying goes, every ash cloud has a silver lining. The cloud provided many scientists with an unprecedented opportunity to research volcanic ash in more detail than they could have dreamt of in a natural, evolving experiment. Also in this week's podcast, Richard Hollingham finds out that river sediment isn't just useless, brown sludge. It turns out that sediment is like a time machine, holding a wealth of information about the past. Chemicals around at any given time eventually build up in river sediments, revealing how the urbanisation of cities like London has changed over time.