Today I'm walking in some beautiful countryside about twenty kilometers from the city of San Francisco. It's very peaceful here with a few wild animals and trees. But in fact I'm standing on something quite dangerous, because I'm walking along the San Andreas Fault. It's a huge fault line which goes right throught the state of California. Now to help me understand the San Andreas Fault. I'm with Claire Hands who is a geologist, and she specialises in the study of earthquakes and in the San Andreas Fault in particular. So Claire, we're stading right on the fault. What would happen if the fault opened right now? Well, if it opened and there was an earthquake, we'd probably fall over. It'd be really difficult to stand up because the ground would be moving. But if we were standing in the countryside like we are now, we'd probably be safer than anyone would be in a city like San Francisco. Right. Because of all the buildings falling down. That's right. And the fault line goes right throught the centre of San Francisco so an earthquake would be really bad news for anyone living here. And that has happend, hasn't it? Yes, the most famous earthquake in San Francisco was in 1906. There have been lots of smaller earthquakers or tremors in California since then but nothing as big as that one. So, it is possible to guess when an earthquake will happen? Scientists and especially geologist would like to be able to do that. We understand a lot about earthquakes and there is equipment which monitors them. We can predict where they will happen and possibly how big the will be. The problem is 'when'. We can't predict when they will happen. If we knew this, we could make a much bigger difference.