This is the River Weaver in rural Cheshire and here a typical riverbank flora of red clover and hogweed, nettle and thistle, plantain and hemp agrimony. But if I wander a little bit further on, it all changes. Gone are those tall and luscious wild flowers native of rich soils. And in their place, scentless mayweed, sea aster with those soft pink daisy flowers and grasses that you’d normally expect to find at the coast. But here we’re fully 30 miles away from the nearest sand dune. What's going on? Alan Titchmarsh heads underground. He's driving around Cheshire but 700 feet underground – right underneath those plants is a salt mine. Salt - essential on your fish and chips and vital for keeping our roads free of ice right through the winter. What’s more, it’s sea salt – 30 miles inland. Those seaside plants up top are here because of the salt in the soil. And all this salt can mean only one thing: this place was once an ocean. 300 million years ago, Britain was underwater! We were flooded by the Zechstein Sea, which was as rich in salt as the Dead Sea, and it covered most of Europe. And this is what nearly the whole of Britain would have looked like: a vast shallow sea, which only our hilltops would have poked through. Slowly but surely the sea dried up, leaving millions of tonnes of salt behind. So, just a small, out-of-place, salt-loving sea aster, can tell you what Britain was like 300 million years ago.