isthmus was never ceded by Spain; rather, it was gradually occupied de facto by the British. The growth of Gibraltar onto the southern half of the isthmus began with the construction of two forts in the northernmost part of the ceded territory. In 1815, Gibraltar suffered a yellow fever epidemic. This led to a Spanish concession so a temporary quarantine health camp could be built on the isthmus. After the plague was over, the British did not remove the camp. In 1854, a new epidemic prompted more health camps, which eventually gained Gibraltar these extra 800 metres of land. The occupation of the isthmus culminated in 1938-1939 with the construction of the aerodrome during the Spanish civil war. The Treaty of Utrecht did not mention British sovereignty over Gibraltar beyond the fortified perimeter of the town as it was in 1713. However the Treaty ceded the town "together with the port, fortifications, and forts thereunto belonging", which included several forts along the line of the current frontier. The United Kingdom further claims title to the southern half of the isthmus based on continuous possession over a long period. The territory north of the current border had been occupied by Spain in the 1730s, during the construction of the so-called Spanish Lines or more properly Lines of Contravallation of Gibraltar. Territorial waters Both the United Kingdom and Spain have ratified the United Nations Convention on the Law of the Sea of 1982, which governs countries' oceanic territorial claims. Both countries made statements regarding Gibraltar in their declarations upon ratification of the Convention. The Spanish statement was: 2. In ratifying the Convention, Spain wishes to make it known that this act cannot be construed as recognition of any rights or status regarding the maritime space of Gibraltar that are not included in article 10 of the Treaty of Utrecht of 13 July 1713 concluded between the Crowns of Spain and Great Britain. Furthermore, Spain does not consider that Resolution III of the Third United Nations Conference on the Law of the Sea is applicable to the colony of Gibraltar, which is subject to a process of decolonization in which only relevant resolutions adopted by the United Nations General Assembly are applicable. The British government responded to the Spanish statement in their own statement: With regard to point 2 of the declaration made upon ratification of the Convention by the Government of Spain,