Greenstone Span

The Greenstone Span is a short aqueduct in the High City of Torrune. It originates from the Carnelian Baths atop the Hart's Hill and runs a few hundred yards to the Blue Pearl Palace atop the Hill of Okhtar, crossing the Tirasval between the two hills and carrying water supplied by the same natural spring which feeds the baths. This water is delivered to the cisterns of the archon's palace, and to the subsidiary water channels which flow down from Okhtar's Hill to feed the Pool of Parth and the Silk Market Cistern in the city below. The proper name of this structure is the Lotharian Aqueduct (after Gideon of the House of Lothar, first Lord of the City, after whom the Lotharian Way below is also named) but it is almost never called this. Its popular name, the Greenstone Span, is universally used instead. 
The main central arch of the Greenstone Span is enormous, over 150 feet wide and nearly as high, crossing the deepest part of the Tirasval (and the Lotharian Way which runs along its center) in a single soaring span. Several smaller arches support either end of the aqueduct as they meet both hillsides. The central arch, like some other prominent edifices in Torrune, was designed with the aid of dwarven architects and is of marvelous workmanship. It has never needed repair or improvement and, many say, never will. However, the stonework of the water channel itself is of less remarkable quality. The source of the aqueduct's popular name is the fact that over time, the lime mortar lining of the water channel will deteriorate, allowing moisture to seep through cracks and porous stone in the channel's bed. This moisture then drips down the sides of the arches and their piers, causing a greenish layer of moss to slowly grow on their surface. Every decade or two, the leakage becomes severe enough that the channel must be re-lined with a fresh layer of mortar. This repair work temporarily cuts back on the mossy growth, but it inevitably returns after a few years. 
Popular belief has it that a narrow secret passageway runs the length of the aqueduct, though the stonework of the watercourse just under the channel's bed, allowing clandestine movement between the archon's palace and the Carnelian Baths. It is probably not surprising that in a place as riddled with underground tunnels as Torrune, every substantial structure in the city will spawn gossip of hidden passages and secret chambers.