been before, or where no-one has reported on the conditions there, is explicitly beyond the limits of a divers experience, but it is done routinely by divers who explore new dive sites. Most of these divers do not experience problems on most of their exploratory dives, and at the end of the dive have extended their experience. Those organisations which train divers at the higher levels of certification are more likely to caution divers to expand their experience gradually, making as few changes at a time as reasonably practicable, and ensuring that they remain competent at all relevant skills within the extended range as it is expanded. Training and safety standards There is a large variability between certification standards, and major philosophical differences regarding the minimum level of acceptable competence. Training in the minimum required skills specified for the certification, splitting certification into multiple courses for maximum diver convenience and agency profit, is common in mainstream recreational diver training, where the diver is responsible for their own safety and the choices they may make, and the agency is protected by legal waivers, and where entry level training typically can be done over about three or four days, with four open water dives worth of experience, and on-line self-study and automated assessment of the theoretical knowledge component. Fitness to dive is self assessed using a questionnaire. This has been shown to be an effective business strategy. Ensuring during the entry level course that the diver is adequately skilled to deal with the range of conditions and equipment likely to be encountered at work is typical of professional diver training where training standards are set out by legislation for workplace health and safety, where entry level training may be full time over a month, with several hours of confined water skills training and practice, a substantial classroom based theory and knowledge section, and about 30 open water dives, with practice and assessment of all critical skills and compulsory medical fitness assessment by a registered diving medical practitioner. A few recreational/technical certification agencies provide entry-level training to intermediate levels. Differences between theory and practice Normalisation of deviance and practical drift. Evolution of standard practice to suit reality, drivers thereof. Hazards Divers operate in an environment for which the human body is not well suited. They face special physical and health risks when they go underwater or use high pressure breathing gas. The consequences