The most celebrated structure at Lothal is a massive trapezoidal basin measuring roughly 218 by 37 metres, cut from baked brick and lined with a carefully engineered inlet and outlet channel. The leading theory holds that Indus Valley merchants used this as a tidal dock, taking advantage of the twice-daily flood from the Gulf of Khambhat to float boats in and out.
Walking along the top of the remaining brickwork gives a good sense of the scale and precision of the original construction. The brick courses are still clearly visible, and the inlet channel that connected the basin to the river can be traced across the surrounding ground. Whether or not you accept the "world's oldest dock" claim, the engineering is genuinely impressive for a civilisation 4,500 years old.