The Polluted Obstacle is an obstacle in BABFT. The river is littered with acid and other pollutants that make a boat slower on water. It also has floating highly explosive barrels that explode when touched. Rusty metal beams stick up from the water.
The acid spots damage the boat on contact, and never dispels. (Until the map gets replaced) The rusty beams act as the average rock that slightly damages weaker part of your boats, and it has a long range.