water and the depth at each end marked. In Russia, measuring the depth of caves by hydrolevelling began in the 1970s, and was considered to be the most accurate means of measuring depth despite the difficulties in using the cumbersome equipment of the time. Interest in the method has been revived following the discovery of Voronja on the Arabica Massif in the Caucasus – currently the world's second deepest cave. The hydrolevel device used in recent Voronja expeditions comprises a transparent tube filled with water, which is coiled or placed on a reel. A rubber glove which acts as a reservoir is placed on one end of the tube, and a metal box with a transparent window is placed on the other. A diver's digital wristwatch with a depth gauge function is submerged in the box. If the rubber glove is placed on one station and the box with the depth gauge is placed on a lower one, then the hydrostatic pressure between the two points depends only on the difference in heights and the density of the water, i.e. the route of the tube does not affect the pressure in the box. Reading the depth gauge gives the apparent depth change between the higher and lower station. Depth changes are 'apparent' because depth gauges are calibrated for sea water, and the hydrolevel is filled with fresh water. Therefore, a coefficient must be determined to convert apparent depth changes to true depth changes. Adding the readings for consecutive pairs of stations gives the total depth of the cave. Accuracy The accuracy, or grade, of a cave survey is dependent on the methodology of measurement. A common survey grading system is that created by the British Cave Research Association in the 1960s, which uses a scale of six grades. BCRA grading system BCRA gradings for a cave line survey Grade 1 Sketch of low accuracy where no measurements have been made Grade 2 (use only if necessary, see note 7) May be used, if necessary, to describe a sketch that is intermediate in accuracy between Grade 1 & 3 Grade 3 A rough magnetic survey. Horizontal & vertical angles measured to ±2.5 °; distances measured to ±50 cm; station position error less than 50 cm. Grade 4 (use only if necessary, see note 7) May be used, if necessary, to describe a survey that fails to attain all the requirements of