Your survey data starts at a singular Zero Datum (ZD) survey station. The depth of the cave is determined from/relative to this unique station. There may be alternative cave entrances below ZD ("deeper"), and some above ZD ("higher"). Cave depth is measured from the highest to lowest survey stations.
All your survey data is entered in a cave surveying program (not in GE), then converted by that software for use in the GE program. The UTM Easting and Northing are the X and Y graph coordinates to locate the point on a suitable plan map, like a topo map. The Z coordinate (relative depth of each survey station) is calculated from the depth assigned to ZD in the cave survey program. Most survey programs have a dedicated file where you store the coordinates (X,Y) and altitude (Z) of the ZD survey station. In Compass this is called a Project or .MAK file.
PM me if this is not clear.
Jim