Definition
Latin. "The high sea" or "the deep sea." A term of old English law denoting the open ocean beyond the limits of territorial waters or coastal jurisdiction. Encountered most often in the phrase *super altum mare* — "on the high seas" — used to establish that an act, crime, or matter of jurisdiction arose outside the territorial reach of any particular sovereign and thus fell under admiralty or maritime jurisdiction rather than the ordinary common law courts.
Why It Matters in Research
The phrase *altum mare* is a jurisdictional marker, not merely a geographic description. When you encounter it in old English cases or pleadings, its function is to place the matter within admiralty jurisdiction — a critical distinction because the common law courts and the Admiralty courts operated under competing authority over maritime affairs for centuries. The presence of *super altum mare* in a pleading signals that the researcher should be looking at admiralty practice, not common law procedure.
Researchers working in pre-nineteenth century English sources will find the term appears in jurisdictional disputes between the Court of Admiralty and courts of common law. The common law courts sought to limit admiralty jurisdiction by insisting that acts done *super altum mare* were cognizable only in admiralty, while simultaneously using writs of prohibition to claw jurisdiction back to common law where any colorable land connection existed. Understanding this tension is essential for reading cases in which the phrase appears.
The term itself does not appear in modern legal usage. It has been fully displaced by "high seas," its direct English equivalent, and by the modern international law framework governing maritime jurisdiction. Researchers working on contemporary maritime law will not encounter *altum mare* in practice but may need to trace how the concept maps onto modern definitions when working through historical materials.
Historical Dictionary Support
All four source dictionaries are in close agreement: *altum mare* means the high sea or the deep sea, and each traces the usage to the same authorities — Coke on Littleton (Co. Litt. 260b) and Hobart's Reports (Hob. 212b). Burrill adds a reference to Brownlow's Reports (Brownl. part 2, 17), extending the citation base slightly. Rapalje & Lawrence offer the most compressed treatment, giving only the definition without citation. No source dictionary supplies meaningful doctrinal context beyond the bare translation and the Latin phrase *super altum mare*.
What the historical dictionaries do not address is the jurisdictional stakes attached to the phrase — the centuries-long contest between admiralty and common law over maritime matters. Researchers relying solely on dictionary definitions will understand what the words mean but may miss why the phrase was legally significant when it appeared in pleadings and early cases.