Solo diving has occurred for millennia as evidenced by artifacts dating back to the ancient people of Mesopotamia when humans dived to gather food and to collect pearl oysters. It wasn't until the 1950s, with the development of formalised recreational dive training, that recreational solo diving was considered dangerous, particularly for beginners. In an effort to mitigate associated risks, some scuba certification agencies incorporated the practice of buddy diving into their diver training programmes. Some divers, typically those with advanced underwater skills, prefer solo diving over buddy diving and assume responsibility for their own safety. Professionally, solo diving has always been an option as it applies to operational requirements and risk assessment. It customarily involves voice or line communications from diver to surface, usually with a diver at the surface on standby ready to assist on short notice. Recreational divers seldom have such options available.