 The study of submerged aircraft lost in lakes, rivers or in the sea is an example from the historical, industrial or modern era.Many specialist sub-disciplines within the broader maritime and underwater archaeological categories have emerged in recent years.Maritime archaeological sites often result from shipwrecks or sometimes seismic activity, and thus represent a moment in time rather than a slow deposition of material accumulated over a period of years, as is the case with port-related structures (such as piers, wharves, docks and jetties) where objects are lost or thrown off structures over extended periods of time.