It is the third-largest natural harbour in the world but even so, it isn't deep enough for modern ships. Falmouth in Cornwall wants to invest £100 million to modernise its ship-repairing docks and facilities for cruise liners.

The project would create hundreds of jobs, protect existing businesses and bring cash-laden tourists into the surrounding area. It depends on being able to dredge the channel into the harbour and that's where the problem lies - to do so would mean digging up rare calcified seaweed called maerl which is protected by law and lies in a special conservation area.

It's a classic stand-off between economic development and protecting the natural environment- now specialist marine scientists have been called in to see whether both sides can be satisfied. Tom Heap gets to grips with rare seaweed and big bucks in Cornwall for 'Costing The Earth'.

Producer: Steve Peacock.