alisdair: what special is the limestone.f the water that comes into the thames is coming from down there. we are taking too much out and we are not allowing it to be this great big, lovely, cool, shading watery sponge that it has always been. this is so much greener than it was just four or five days ago. it was just ocher. if you're kind of used to the thames in london, which obviously it's such a big, iconic, famous river, if you've visited london and want to know where it starts, it starts there. so if we just come dowhere. there you go. that's the start of the thames. it is maybe a little bit underwhelming, but this is the source of the thames. malcolm: in normal times, what would we see? alisdair: well, in normal times, you would see this. this source does dry up each year. pretty much. but you can come here when you've got normal levels of rain and you've had a normal wet winter. and we'd be still in a pond. -- and we would be stood in a pond. malcolm: we headed east along the river's normal course. deep water? there w