Somewhat complex trained reflex behaviors may be developed by experienced divers, involving breathing control and BC gas management during depth changes, which allow them to remain neutrally buoyant from minute to minute during a dive, without having to think much about it. Skilled scuba divers may be identified by their ability to maintain constant depth in horizontal trim, without fin use. Ease and accuracy of buoyancy control is affected by awareness of changes of depth. Precision control is relatively easy while there is a clear visual reference, but more difficult when the only reference is instrumentation. The most difficult circumstances for most scuba divers is during ascent in low visibility in mid-water without an ascent line, a time when depth control is most important for safety.