The following explanation was found and posted by a Russian learner of English for the benefit of his fellow learners from Russia and speakers of other Slavonic languages.

I found a rule: "the" must be put before the names of seas, oceans, rivers, channels, bays, archipelagos, chains of mountains, lands (The Netherlands);

"the" is never put before the names of lakes, countries (except the names with common noun: The United Kingdom, The Russian Federation etc.), cities (towns), mountains, islands.

A good set of rules, except in respect of bays, e.g. Botany Bay (Australia), Herne Bay [England], Poole Harbour (England: more of a bay than a harbour)

Capes depend on the form of the name, e.g. Cape Horn (South America), the Cape of Good Hope (South Africa)

Regions comprising various countries generally take the definite article, e.g. the Baltic [Pribaltika], the Balkans, the Middle East, the Levant [ust.]

Continents and analogous entities do not take the definite article, e.g. Europe, South America, Polynesia, South East Asia

Note also "the Arctic" and "the Antarctic" but "Antarctica".

Islands, mountains etc. with "of" in the name take the definite article: e.g. the Isle of Man, the Isle of Wight, the Isles of the Dead. [mythology], the Mount of Olives [biblical]

Groups of islands take the definite article.

e.g. the Canaries, the Orkneys, the Channel Islands [Normandskie o-va], the British Isles, the British Islands [NB this last one used only in legal texts to indicate the inclusion of the Channel Islands (Interpretation Act 1978)]

Also e.g. "Malta" but "the Maltese Islands"

Some country names took the definite article until independence, e.g. the Gambia, the Lebanon, the Ukraine

Note also "the Crimea".

"Chains of mountains" isn't wrong but just sounds odd. We say "mountain ranges".

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