During construction they dug into a section of Qing-dynasty culvert.
Tips
memory
暗 (àn, 'hidden / dark') + 渠 (qú, 'ditch / channel'). The opposite is 明渠 (míngqú, 'open channel'), water that flows out in the open. The 暗 / 明 pair shows up across Chinese for 'hidden vs visible' (暗号 vs 明号, 暗箭 vs 明枪).
