It may also be used to refer to aspects of maintenance which are not specifically covered by the technical departments.
[6]: 15 The underside of the hull is an overhead environment with no direct vertical access to the surface.
As such it constitutes an entrapment hazard, particularly under large vessels where it may be too dark due to low natural light or turbid water to see the way to the side of the hull.
The bottom of the largest ships is mostly flat and featureless, exacerbating the problem.
Only surface-supplied diving is authorised for this work in most jurisdictions, as this not only secures the diver's breathing gas supply, but also provides a guideline to the exit point.