'Hydra' by Robert Swindells, is an excellent book about Ben & Midge, a boy and a girl from the local school investigating the mystery of a crop circle that has recently appeared at Cansfield farm. At first it seems to be nothing, but after a midnight break in to Cansfield, they begin to realise the circle was created by alien jellyfish, seemingly harmless, untill they gradually mutate in to a much more dangerous lifeform, that would throw Midge, Ben and the rest of the world into danger IF they are allowed out...