The Barbegal Aqueduct included a watermill complex with water cascading through a total of sixteen wheels. It may have been "the greatest known concentration of mechanical power in the ancient world"

Huh. That's interesting. Would you like to talk about Roman engineering, or watermills, or what? There's a lot of places a conversation could go from that link, but I'm interested in where you would like the conversation to go.

Not a whole lot there. From another article about it:

Originally, it was believed that the mills of Barbegal were built at the end of the third century CE, when Arles was the residence of the Emperor Constantine, for the production of flour for the imperial administration, the army, and the city (10). The mills were previously used to illustrate a profound social change: With Christianization, the end of slavery liberated the inventive capacities of engineers and favored the use of natural energy, in this case, hydropower, to produce flour. However, excavations in the early 1990s led to the abandonment of this hypothesis [...]

Today, nearly 80 years after the first excavation of the complex (1937–1939), key questions still remain open: What was the destination of the large amount of flour produced by the mill complex? How did it function and for how long was it in use? And did mill complexes of this scale spread over the Roman Empire? These questions have remained unsolved because the ruins are only fragmentary, excavation records are incomplete (10), and only few other industrial sites of comparable scale are known in the Roman Empire.

[...]

A comparison of the microstratigraphy of coeval fragments (Fig. 3) reveals clear evidence of maintenance of the Barbegal mills and shows that wooden structures were replaced approximately every 5 to 10 years, dictated by the rate of mold and decay. Periodic maintenance is known from medieval mills and was typically required at these intervals (27).

[...]

Because of the vicinity of the Roman city of Arelate, it was originally proposed that the mills of Barbegal provided flour for the entire population of this city (5, 12). However, the annual interruption of mill activity revealed by the stable isotope data is difficult to reconcile with a continuous year-round demand for flour to a city. Transport of large quantities of flour over long distances or months-long storage seems unlikely because of spoilage. An alternative explanation, that the mills were used to produce flour for the army, is unlikely because no large military concentration is known from the area for the period of activity of the mills.

The proximity of the major ports of Arles and Fossae Marianae (Fig. 1A) suggests an alternative destination of the massive amounts of flour produced by the Barbegal complex: It may have been used for the production of ship’s bread (panicus panis) meeting the needs of ships that frequented these ports. Panis nauticus, equivalent to panis militaris or buccelatum (28–30), was a major staple aboard ships. After it was double baked, it was suitable for long-term storage, just like ship’s bread typically used in later periods. A similar situation has been proposed to explain the large number of bakeries in Ostia, the harbor city of ancient Rome (31).

[...]

The use of the Barbegal complex seems to have ended after a sudden exposure of the structures to sunlight, leading to the formation of tufa-like calcite capping the dense crystalline deposit in the still working flumes, before the mills were finally abandoned (Fig. 4, A and A1). In these last porous deposits, imprints of fragments of worked wood and other debris are common, suggesting that the active operational period of the mills ended in a nonstructured manner by lack of proper maintenance. This event is dated to the early third century CE (11). At that time, an economic crisis associated with military and political turmoil affected the Roman Empire (33), which likely led to the abandonment of this industrial mill complex.

There was a TV show called Connections that began an episode on this site (as noted on Wikipedia, apparently). I highly recommend checking it out on youtube!