A national sea border is usually understood as the line of the shore. In practice, however, this is not accurate. According to international law, a littoral state has rights stretching out into the sea, constituting partial sovereignty over a series of areas distanced from the shore’s baseline, beyond which marine expanses are classified as “high seas” and are common to all. The Black Sea, for instance, has no high seas and is entirely divided up by the countries that surround it. Depth, volume, surfaces, and airspaces are all involved here in complex concatenations. In the various sea zones, rights to subsurface or other potential resources (oil, gas, fishing, mineral extraction) cannot easily be separated from the armed naval, submarine, and air forces that protect them and provide security for coastal states. Recently Russia has dramatically expanded its maritime control area by virtue of the takeover of Crimea, to the detriment of Ukraine’s jurisdiction in the Black Sea.

The shoreline is not an absolute border on the landed side either. Ports have enclaves that are subject to special regulations, such as quarantine areas, holding spaces for immigration, consulates, and warehouses belonging to foreign countries and/or multinational companies. In many port cities there are a-legal spaces too, which are not subject to normal state sovereignty because they are hidden. One such space on the Black Sea is the catacombs of Odessa, a vast warren of underground passageways and chambers that have never been fully mapped. The limestone used for building houses was quarried the seams lying directly under the city. No restrictions were placed on extraction, with the result that the whole city sits on a labyrinth of underground tunnels. These link with pre-existing caves formed by the action of the sea on the cliff. Reaching a depth of sixty meters in places and with over one thousand known entrances, passages snake for some 2,500 km beneath the city. Further widened by smugglers and subject to resumed mining in the twenty-first century, the catacombs continue to expand. Thus, spreading on both the sea and land sides of the border—but in fact making up its very three-dimensional multiplicity—there were (and still are) spatial pockets or pools whose subsurface volume was essential to what they were.

Historically, the catacombs were used for two border purposes: smuggling and storage of clandestine goods, and secret revolutionary organization and resistance. Both of these purposes literally and metaphorically undermined the rule of the government on the surface. These political and economic functions have continued into the present: resistance to Fascist occupation during World War II, the presence of criminal gangs and contraband in the Soviet era, the training of self-defense units in Ukraine during the fighting of 2014 and 2015—all of these have left residues. The two kinds of depth spaces, those under the surfaces of the sea and of the land, are not entirely distinct but form a complicated zone of family resemblance: that is, they are connected by a number of overlapping similarities.

One of the complexities of the sea border is that it has involved repeated trials of strength between the interests of different nations and with international law. The result has been the appearance of ambiguous objects. The Black Sea is unlike another contested sea, the Arctic, where the problem is rather that its physical nature is ambiguous: the Arctic melts; it freezes and appears like land; parts break off and move around (Steinberg, Tasch, and Gerhardt 2015, 42). In the Black Sea, the physical geography of the border zone has remained more or less constant, but what changes and gets contested is what certain geographical objects are held to be. Answers to questions such as “is it an island or a (mere) rock?” are politically consequential to the extent that they define maritime sovereignty, which is why they are hotly debated in other theaters of contemporary global tension such as the South China Sea. In a multinational border region, especially one at war (as the Black Sea currently is and has often been in the past), negotiating what certain key objects can be said to be is the most slippery problem of all.

The infrastructure of a sea border and its relation to the boundaries of state sovereignty are not simple matters, especially in the case of ports. Here the state border is still conceptually a line; it appears as such on larger-scale maps and travelers may imagine a line that they cross at immigration control. Yet notwithstanding the “fundamental divide between land and water that underpins modern notions of sovereignty” (Steinberg, Tasch, and Gerhardt 2015, 41), at ports extraterritorial and extramaritime spaces appear on both sides of the shoreline. Around the line itself, jetties, harbors, breakwaters, watchtowers, and lighthouses create a series of complex marine volume-spaces that de-compose the linear border and turn it into a zone. The case of Odessa has shown that the idea of a warren can apply across this zone. It is a metaphor that applies as well to the channels of the Soviet submarine fleet (the bearer of much contraband) as to the network of tunnels under the city, that home of the clandestine. The warren creates a potentiality for those hidden flows that are part of the multidimensionality of borders.