A ballast pond was a construction found in shipyards during the age of sail. The feature was often a prominent part of the large dockyards of the Royal Navy, where large numbers of warships would be laid up 'in ordinary' during periods of peace, when the navy retained only some of its ships in active commission. Ships in ordinary often had their stores and guns removed, as well as their masts. The ballast was then dumped in the pond. Over time the tidal action of the water flowing in and out filtered the ballast in the pond, removing any impurities. The filtered ballast could then be recovered and returned to the dockyard to replace ballast in other ships being put in ordinary, or that otherwise needed extra ballast.