Rusticle

right|thumb|Detached rusticles below port side anchor of the Titanic|RMS Titanic A rusticle is a formation of rust similar to an icicle or stalactite in appearance that occurs deep underwater when iron-eating bacteria attack and oxidize wrought iron and steel. They are seen on underwater photographs of shipwrecks, such as the RMS Titanic and the German battleship Bismarck. They have also been found in the #3 8-inch gun turret on the stern of the USS Indianapolis. The word rusticle is a portmanteau of the words rust and icicle and was coined by Robert Ballard, who first observed them on the wreck of the Titanic in 1986. Rusticles on the Titanic were first investigated in 1996 by Roy Cullimore, based at the University of Regina in Canada. A previously unknown species of bacteria living inside the Titanics rusticles called Halomonas titanicae was discovered in 2010 by Henrietta Mann.

Rusticles can form on any submerged steel object and have been seen on other subsea structures such as mooring chains and subsea equipment. They form more rapidly in warmer climates and can form in water with little to no dissolved oxygen.