score:8
It seems likely to me that Clement was referring to the "Qurnah Disaster", named after Al Qurnah in Iraq, about 75 km north of Basra:
In 1855, Al Qurnah was the site where local tribes attacked and sank a convoy of a ship and rafts carrying 240 cases of antiquities discovered by Victor Place's mission to Khorsabad, Rawlinson's to Kuyunjik and Fresnel's to Babylon. The loss of priceless antiquities was a notable disaster for those researching the antiquities of the region. Subsequent efforts to recover antiquities lost in the Qurnah Disaster, including a Japanese expedition in 1971-2, were largely unsuccessful.
The loss amounted to decades' worth of collections by several different archaeological expeditions.
A more detailed account is here; the report of the Japanese recovery effort is also interesting.
I don't know how widely known this story was a few decades later, when Clement wrote her book. I don't believe it was widely published until the 20th century.