Beneath the Black Sea’s surface waters lies not a hidden cavern but a different chemical environment: a vast body of water that is almost devoid of oxygen, separated from the upper layer by major differences in salinity and density. In this environment, the wood of some shipwrecks can remain surprisingly well preserved for centuries or even millennia, protected from many of the organisms that rapidly destroy it in other seas.
The most commonly cited threshold is around 150 metres, but it should not be read as a line drawn with a ruler. Offshore, oxygen-poor water may begin at roughly 100–150 metres; near the continental shelf and in certain transition zones, the boundary may be deeper or shallower. For archaeologists, not only depth matters, but also the local circulation of water, sediment and density currents.
Why the Black Sea has oxygen-free deep waters
The Black Sea is almost enclosed in relation to the world ocean. It connects to the Sea of Marmara through the Bosporus, but this exchange of water does not effectively mix the entire water column. Less salty water reaches the surface, supplied in part by the Danube and other rivers. Below it, saltier, denser water enters from the Mediterranean.
The difference in density acts as a stable barrier. The lighter water remains on top, while the heavier water stays beneath it. Winds and storms can churn the surface intensely, but they cannot adequately ventilate the very deep waters. Oxygen enters the sea chiefly through contact with the atmosphere and through photosynthesis by organisms in the sunlit zone. Organic matter sinking from the upper layers is decomposed, and this process consumes the available oxygen.
The result is long-term stratification. More than four-fifths of the Black Sea’s volume lies in waters with no dissolved oxygen or almost none. Hydrogen sulfide also accumulates at depth, a compound associated with the smell of boiled eggs, although this is not something tourists on the beach “smell” in coastal water.
To complement the information above, see also Beneath Antarctica’s ice is not a “frozen” world: what is millions of years old and what is still moving, an article that explores an aspect directly related to this subject.
Three layers of the Black Sea, not a single “dead zone”
| Layer | What characterises it | What it means for a wooden shipwreck |
|---|---|---|
| Oxygenated surface waters | They contain dissolved oxygen and support most visible marine life. | Exposed wood can be attacked by organisms, including species that bore tunnels into it. |
| Suboxic zone | This is the transition in which oxygen has fallen to very low levels, before hydrogen sulfide becomes dominant. | Preservation may be better than at the surface, but conditions are variable and do not provide the same protection everywhere. |
| Anoxic, sulfidic waters | Dissolved oxygen is absent and hydrogen sulfide is present. | Many aerobic organisms that accelerate wood degradation cannot live here; some shipwrecks retain their shapes unusually well. |
The proper term for the Black Sea’s deep waters is often euxinic: an oxygen-free environment containing hydrogen sulfide. The expression “dead zone” is useful only up to a point. There are no fish or ordinary marine animals in these waters, but the environment is not devoid of life. Adapted bacteria use other chemical reactions to obtain energy, including reactions involving sulfur and nitrogen.
Therefore, the lack of oxygen does not halt all biological processes. It radically changes which organisms can live there and how quickly organic materials degrade. This difference is essential for shipwrecks.
What actually protects the wood of a sunken ship
In most seas, a wooden vessel that reaches the seabed quickly becomes both habitat and food. Bacteria, fungi and, above all, marine wood-boring organisms can weaken hull planking, decks and masts. In oxygenated water, a vessel can relatively quickly lose precisely the elements that reveal its original form.
In the Black Sea’s anoxic layer, many of these organisms cannot survive. As a result, exposed wood can remain recognisable for far longer than would be normal in a well-oxygenated marine environment. This preservation is all the more valuable to archaeology because a shipwreck no longer conveys only its cargo—amphorae, pottery or metal objects—but also information about how the vessel was built: the outline of the hull, the position of the mast, the rudder, rowing benches or deck details.
However, the formula “no oxygen = intact wood forever” is wrong. Anaerobic microorganisms can also act on wood, while sediment, corrosion of metal parts, currents and human intervention can affect a wreck. The absence of oxygen greatly reduces certain forms of degradation, but it does not turn the Black Sea floor into a perfect freezer.
The example that changed how we view Black Sea shipwrecks
In October 2018, the Black Sea Maritime Archaeology Project team announced the dating of a Greek merchant ship discovered in Bulgarian waters of the Black Sea. A wood sample indicated an age of at least 2,400 years, dating to around 400 BC. The vessel lies at a depth of more than 2,000 metres, in an anoxic environment.
The importance of the discovery lies not only in the vessel’s age. The ship retained structural elements that are normally among the first features lost from a wooden wreck. Researchers were able to compare its form with depictions on ancient Greek vessels, providing a rare link between historical images and a real seafaring object. Details of the project and its dating were presented by the University of Southampton, one of the institutions involved in the research.
The project mapped more than 2,000 square kilometres of seabed and identified dozens of wrecks from different periods. Not all were at extreme depths, and not all were equally well preserved. This nuance matters: the phenomenon is not an automatic promise made to every vessel that sinks in the Black Sea.
Why 150 metres is only a guide
The simplified picture is that “the oxygen-free zone begins below 150 metres”. In practice, the interface between oxygenated and anoxic waters changes according to geographical position, seabed relief and local dynamics. In certain parts of the continental shelf, researchers have found relatively well-preserved wooden wrecks at only 100–115 metres.
The proposed explanation is that internal waves and density currents can periodically push oxygen-poor water towards shallower areas. These movements are invisible from the surface, but they can affect whether the larvae of wood-attacking organisms succeed in colonising a wreck.
As a result, two wrecks at similar depths may be in very different states of preservation. One may be protected by local chemical conditions, while another may be exposed to organisms, currents or bottom-fishing activities. A study in the Black Sea’s southern sector documented damage caused by bottom trawling, including in areas with high archaeological potential.
What the phenomenon means for Romania’s coast
Romania has access to the Black Sea, but that does not mean the anoxic zone is accessible from the beach or begins a short distance from shore. The tourist coastline, ports and most ordinary maritime activity take place in oxygenated surface waters. Very deep water and the anoxic environment are an oceanographic reality of the basin, not an attraction that can be seen with the naked eye from Constanța or Mamaia.
Nevertheless, the phenomenon is relevant to the region’s heritage. The Black Sea has been an important trade route for Greek, Roman, Byzantine, Ottoman and modern communities. A shipwreck more than 100 years old can fall within the category of underwater cultural heritage under the 2001 UNESCO Convention, a convention accepted by Romania on 30 July 2007. The basic principle is protection and responsible research, not the improvised recovery of objects.
This is also why striking images of Black Sea shipwrecks should be viewed as scientific documents, not as invitations to explore independently. The most important sites lie at depths that require underwater robots, sonar mapping, specialist teams and conservation procedures. Raising old wood from the water can trigger rapid degradation if the material is not treated properly.
Not only shipwrecks are “archived” at depth
The Black Sea’s anoxic waters and seabed sediments also preserve traces of environmental change: pollen, organic remains, deposits brought by rivers and clues to the evolution of coastlines. Research therefore does not focus exclusively on ships. It seeks to reconstruct how sea levels, shores and human communities have changed over millennia.
This combination makes the Black Sea unusual: it is at once a marine system with extreme chemistry and a historical archive. While at the surface it functions as a familiar sea for navigation, fishing and tourism, its depths preserve conditions that can conserve fragments of the old world better than many other seas.
The essential point
Black Sea shipwrecks do not endure because the water has mysterious properties. They can be preserved because the deep layers are isolated from oxygen at the surface, and the organisms that consume wood are far fewer or absent. But the effect depends on depth, exact location, water dynamics and human intervention. The Black Sea is not a perfectly sealed museum; it is a rare environment that gives archaeology an exceptional chance to see ancient ships almost in the form in which they reached the seabed.
Frequently asked questions
At what depth is there no longer oxygen in the Black Sea?
There is no single depth that applies everywhere. In general, the transition to waters that are very poor in oxygen occurs at roughly 100 to 200 metres, and many explanations use the benchmark of around 150 metres.
Is there life in the Black Sea’s oxygen-free zone?
Yes, but not ordinary marine life such as fish or most bottom-dwelling animals. Bacteria adapted to oxygen-free conditions use chemical reactions based on sulfur and nitrogen to obtain energy.
Why are wooden shipwrecks better preserved there?
The lack of oxygen prevents or limits aerobic organisms, including many of those that bore into and consume wood. As a result, degradation can be much slower than in oxygenated marine waters.
Does that mean every Black Sea shipwreck is intact?
No. A wreck’s condition also depends on depth, currents, sediment, microorganisms, corrosion and activities such as fishing with bottom gear. The transition zone between water layers is especially variable.
Can hydrogen sulfide from the depths be smelled on Romania’s coast?
Normally, no. The sulfidic layers are in deep waters, below the oxygenated surface zone. Local smells along the coast may have other causes and do not prove that anoxic water has reached the beach.
