Ships crossed it. Empires controlled it. Traders depended on it. Navies fought over it.
Today, the sea is becoming something more complicated.
It is simultaneously a source of energy, food and minerals, a space for communication cables and renewable energy, an environmental system requiring protection, and one of the principal arenas of geopolitical competition.
This creates a difficult question.
Where does legitimate national interest end and the gradual creation of geopolitical facts begin?
The question has become particularly relevant in the Eastern Mediterranean. But it is certainly not confined to our region.
From the South China Sea to the Arctic and from the Pacific to the Mediterranean, similar tensions are appearing in different forms.
The Eastern Mediterranean is only one example
The latest dispute between Greece and Turkey over marine parks illustrates the problem particularly well.
Turkey recently established two national marine parks, one in the northeastern Aegean and another in the Eastern Mediterranean.
Greece argues that parts of these areas extend beyond Turkish territorial waters, including into areas of the high seas and the Greek continental shelf, and therefore cannot produce the legal effects claimed by Ankara.
Behind the environmental terminology lies a much older disagreement about maritime jurisdiction.
Turkey and Greece have fundamentally different views regarding the maritime rights generated by islands, the continental shelf and Exclusive Economic Zones.
This dispute is connected to a much larger geopolitical picture involving Cyprus, Kastellorizo, energy exploration, electricity interconnectors and competing interpretations of maritime space.
Marine conservation therefore enters an already crowded geopolitical landscape.
That does not mean environmental protection is merely an excuse.
It means that environmental policy and geopolitics can exist in the same place at the same time.
Drawing lines on water
There is something unusual about maritime politics.
Most of the boundaries we discuss cannot actually be seen.
There are no walls across an Exclusive Economic Zone.
There are no fences marking the continental shelf.
A fishing vessel cannot look at the water and identify where one legal jurisdiction ends and another begins.
The boundaries exist on charts, in treaties and in international law.
Yet those invisible lines determine enormous interests.
They can influence who fishes.
Who explores for energy.
Who develops offshore wind.
Who conducts scientific research.
Who lays cables.
Who exploits seabed resources.
And increasingly, who claims authority to protect particular parts of the marine environment.
This makes maps politically powerful.
A line drawn on a map may appear technical.
It rarely is.
The South China Sea
Nowhere is this more evident than in the South China Sea.
Several states have competing maritime and territorial claims there. China has advanced extensive claims, while the Philippines, Vietnam and other countries maintain competing rights and interests.
The dispute is not simply about empty water.
The region contains major shipping routes, fisheries and potential energy resources. It is also strategically important to the military balance in Asia.
Environmental questions have become part of this dispute as well.
Coral reefs have been damaged. Artificial islands have been constructed. Fishing pressures are severe. Different governments accuse one another of harming fragile ecosystems.
The 2016 South China Sea arbitration is particularly instructive because its findings were not limited to maritime entitlements. The tribunal also found breaches of obligations relating to protection of the marine environment.
This reveals an important principle.
International law does not force us to choose between maritime rights and environmental responsibility.
States have both.
The Arctic is changing
The Arctic presents another version of the same problem.
Climate change is transforming the physical geography of the region.
As ice retreats, previously difficult waters become increasingly accessible.
New shipping routes may become commercially attractive. Resources become easier to reach. Strategic competition increases.
Russia has enormous Arctic interests. Canada, the United States, Norway and Denmark also possess significant rights and responsibilities there, while China describes itself as a stakeholder in Arctic affairs.
The environmental stakes are enormous.
The Arctic is one of the planet’s most fragile ecosystems.
Yet the same waters that require extraordinary protection are becoming increasingly important for shipping, energy, minerals, security and military strategy.
Once again, conservation and geopolitics meet.
The Pacific and the seabed
Look further south and another contest is developing.
The deep seabed contains minerals that governments and companies increasingly regard as strategically important.
Polymetallic nodules contain metals that can be used in batteries, electronics and advanced technologies.
Supporters of deep-sea mining argue that these resources could become important for the energy transition and reduce dependence on concentrated terrestrial mineral supplies.
Opponents warn that humanity understands far too little about deep-ocean ecosystems to begin industrial extraction safely.
The debate is therefore environmental.
But it is also geopolitical.
Critical minerals are becoming an important element of strategic competition between major powers.
Control of supply chains matters.
Technology matters.
Access matters.
And suddenly parts of the ocean floor that most people will never see become politically important.
The quiet importance of cables
There is another part of the sea that receives considerably less public attention.
Undersea cables.
Much of the world’s international digital communication travels through fibre-optic cables lying on or beneath the seabed.
Electricity interconnectors are also becoming increasingly important as countries connect energy systems and expand renewable generation.
These cables cross maritime jurisdictions.
Their routes can therefore intersect with unresolved disputes.
A project that appears to be purely technical can quickly acquire geopolitical significance.
Who surveys the seabed?
Whose permission is required?
Which maritime jurisdiction applies?
Who protects the infrastructure?
And what happens if a cable is damaged deliberately?
The modern seabed is becoming infrastructure.
That changes maritime politics profoundly.
The danger of creating normality
There is another dimension that deserves attention.
States do not always need dramatic confrontations to strengthen a geopolitical claim.
Sometimes repetition is more effective.
Publish a map.
Issue a navigation notice.
Conduct research.
Regulate fishing.
Establish an administrative zone.
Announce an environmental programme.
Repeat the terminology.
None of these actions necessarily determines sovereignty or maritime jurisdiction under international law.
But collectively they can create a perception of normality.
And perceptions matter in international affairs.
This is why governments must pay attention not only to dramatic violations but also to gradual attempts to establish facts through administrative practice.
International law should not be replaced by repetition.
But environmentalism must not become the victim
There is, however, an important danger on the other side.
We should not begin treating every marine protected area as a geopolitical conspiracy.
The oceans genuinely need protection.
Overfishing, plastic pollution, warming waters, acidification, biodiversity loss and destruction of marine habitats are real problems.
Humanity needs considerably more international cooperation at sea, not less.
Marine protected areas are an essential part of that effort.
The international community is also developing stronger mechanisms for protecting biodiversity in areas beyond national jurisdiction.
This is an important achievement.
The difficulty is ensuring that environmental protection remains compatible with international law and does not become an instrument for advancing unrelated territorial or jurisdictional claims.
Conservation requires legitimacy.
And legitimacy requires cooperation.
The EEZ matters
The Exclusive Economic Zone is particularly important in understanding this new maritime world.
Under the modern Law of the Sea, coastal states may establish an EEZ extending up to 200 nautical miles, subject to geographical circumstances and the rights of neighbouring states.
Within it, they possess important sovereign rights relating to natural resources and certain forms of economic activity.
They also possess jurisdiction in areas including marine scientific research and protection of the marine environment.
This combination is increasingly important.
The EEZ is therefore not merely about oil and gas.
It is about fisheries.
Renewable energy.
Environmental protection.
Scientific research.
Marine resources.
And increasingly, the management of an entire maritime space.
Where neighbouring claims overlap, however, drawing a line on a national map does not magically settle the matter.
Delimitation requires international law, negotiation and, where states agree, peaceful judicial settlement.
Small islands, enormous consequences
One of the fascinating features of maritime geopolitics is that very small pieces of land can have very large consequences.
An island may contain only a few hundred inhabitants.
Yet its location can affect maritime claims covering thousands of square kilometres.
This is one reason islands have become so important in disputes from the Mediterranean to Asia.
But international law cannot simply be reduced to the proposition that large countries receive large seas and small islands receive none.
Nor can every dispute be solved by drawing automatic geometric lines.
Geography matters.
Law matters.
Previous agreements matter.
Special circumstances may matter.
And peaceful settlement matters most of all.
A global challenge
What we are witnessing is not simply a collection of unrelated maritime disputes.
Something larger is happening.
Humanity is moving more economic activity into the sea.
We are building offshore wind farms.
Connecting electricity grids.
Laying ever more important communication infrastructure.
Searching for minerals.
Protecting biodiversity.
Expanding aquaculture.
Exploring the seabed.
Developing autonomous maritime technologies.
And continuing to depend on shipping for global trade.
The sea is therefore becoming more valuable.
And what becomes more valuable becomes more contested.
The choice ahead
There are two possible ways to respond.
The first is to treat the oceans as the next great territory to divide, exploit and militarise.
That road is already visible.
The second is more difficult.
It requires states to defend legitimate rights while accepting that oceans cannot be governed sustainably through unilateral power alone.
International law is imperfect.
Negotiations are slow.
International institutions frequently disappoint us.
But the alternative is a maritime world in which the strongest actor simply draws the largest map.
That would be dangerous not only for smaller states.
It would ultimately be dangerous for everyone.
The challenge of this century therefore extends far beyond deciding who owns what beneath the water.
We must determine how sovereignty, sovereign rights, environmental responsibility, economic development and global commons can coexist.
The lines on our maritime maps will continue to matter.
But the future of the oceans may depend even more on something those maps cannot show:
Οur willingness to recognise that protecting legitimate national interests and protecting a shared planet do not have to be opposing choices.
- Dean Koyionis writes on international affairs, geopolitics and contemporary global challenges, with a particular interest in the forces shaping societies, cooperation and international relations.