Beyond the occasional shriek from a pointing child, the crocodile enclosure at Johnsons Zoo is normally a place of calm.Something about these prehistoric beasts demands reverence and tends to make visitors emit tiny gasps of awe and speak in cathedral tones.It was like this at lunchtime on Thursday as small groups filed along an elevated walkway in a converted cattle barn, peering over a metal barrier (not unlike the kind that separate crowds from red-carpet celebrities) down at the pit 15ft below.But at just after 1pm something happened at the zoo near Huntingdon in Cambridgeshire that had the effect of contracting time and suspending the present. Something unusually horrifying.Suddenly, a boy of three was in the enclosure's shallow pool. What's more, the crocodiles he had been eyeing excitedly moments earlier were now eyeing him and slithering through tropical vegetation in his direction.Forget every parent's nightmare – who would include this among the litany of their worst fears?Some who were there recalled a fleeting sense of dislocating confusion. What were they seeing exactly? How could a child have fallen? It didn't seem possible.It was by no means immediately apparent, not to everyone, that the toddler hadn't fallen. The reality was darker still. A man with learning difficulties is alleged to have picked him up and thrown him over the 4ft barrier. It is thought the toddler landed on concrete before rolling into the water. Pictured: Johnson's of Old Hurst, the crocodile farm in Cambridgeshire where a boy, three, was left in critical condition after being mauled by a crocodile (File image) Zoo owner Andy and wife Tracey are pictured transporting a 400lb crocodile in 2009All at once, silent disbelief gave way to sounds of panic. A punchy cry. A prolonged, high-pitched wail that one witness said, 'We'll never forget'. Desperate entreaties for someone to do something. And fast. By now this was turning into a horror show. One or more of the crocodiles was beginning to attack the child.Mercifully, someone did do something. Zoo owner Andrew Johnson's wife Tracey, who had celebrated the birth of her first grandchild days earlier, sprang into action.Without hesitation, 55-year-old Mrs Johnson – whom one friend described yesterday as a 'special, steadfast, old-fashioned heroine' – climbed into the pen, waded to the boy and 'swooped down to pull him to safety'.She is then thought to have escaped through a keepers' exit.Chris Newman, director of the National Centre for Reptile Welfare, said: 'Her actions are nothing less than what I would expect of her. She is an incredible lady. It was a very brave thing to do.'Yesterday, the boy was in a critical but stable condition at Addenbrooke's Hospital in Cambridge. Among his injuries are thought to be a broken arm and pelvis.A 30-year-old man arrested at the scene, meanwhile, was released on bail until September 18 after being assessed as 'not fit for interview'.It is thought he was on an organised trip with other vulnerable adults and at least two carers. Pictured: A crocodile in its enclosure at the Johnsons of Old Hurst zoo in Cambridgeshire