Crisscrossed by a maze of waterways, some navigable by small crafts, the terrain is only a few meters at most, above sea level. Go back in time, less than 2000 years ago, there was little, if any, land visible. The plain contains great depths of river-borne alluvium - sediments carried down over time from inland mountain ranges such as the Taihang, even the yellow soil of Shaanxi. Gradually, deposition led to marshes, and eventually land appeared.– This process of sedimentation is still going on beyond the Bohai Sea's present-day shores. Over time, despite this "new" land not particularly suited for large urban settlements, many of the meandering streams were straightened and land reclaimed for farming around small villages. Fishing was important along with salt beds, the latter a significant factor for Tianjin's historic strong economic growth. However, some industrialization also appeared during the early 20th century, constructed with sadly little concern for the local environment. Areas of the wetlands degraded.