Abū Bakr Muḥammad al-Karajī, the 11th century CE mathematician who was active in Baghdad during the Buyid era, wrote several groundbreaking mathematical texts on algebra and other topics. Remarkably, Karajī’s surviving works include, as well, Inbāṭ al-miyāh al-khafīya, or “The Extraction of Hidden Waters” – a text devoted to the construction and upkeep of the underground water channels that have been used for irrigation in Persia and neighboring regions since ancient times. Composed in Arabic, the work appears to postdate Karajī’s fruitful residence in Baghdad and is dedicated to Abū Ghānim Maʿrūf ibn Muḥammad (active first quarter of the 11th century CE), a statesman and belle-lettrist of whom little is known, but who spent a number of years as vizier in the Ziyarid realms to the south of the Caspian Sea. Conceived as a comprehensive manual, Inbāṭ al-miyāh al-khafīya includes sections on geology, botany, surveying technology, and legal matters, as well as providing discussions of the practical issues connected with the construction and ownership of subterranean aqueducts. A notable feature of Karajī’s text on aqueducts is the presence of non-Arabic terms that appear in various parts of the text. Many of these words, such as place names, the names of plant species, and common loan-words for terms such as silk and some tools (such as the compass) would have been in wide circulation in Karajī’s Baghdad. However, more notable cases of multilingualism in the text include Karajī’s use of the name of non-lunar months, as well as a handful of technical terms that appear to refer to the structure of the aqueducts, themselves – the latter group containing terms whose meaning can at best be surmised based on their context. This paper offers a survey of the loan-words in Karajī’s Inbāṭ al-miyāh al-khafīya, in order to examine the linguistic background of Karajī’s intended audience, and to highlight the multicultural nature of his practical manual on aqueducts.