The pond is lined with black plastic but the pond is supposed to look like a natural jungle pond (ideally a peaceful zen-like pond) which you might stumble upon while hiking in the jungle so the black plastic needs to be hidden. We found out after we had all the rocks in place that one side of the pond was a little high so it was difficult to hide the black plastic on that side with rocks. We also realized that the best way to hide the plastic is to have a shelf of rocks a little way below the top level of the pond. We are achieving this by pulling out one layer of sand bags on the high side. It is also necessary to back fill dirt very close to the edge so that we can root plants that will drape over in places and also look natural. The plastic will loop up a little behind the low level of rocks before we back fill with more rocks and dirt. This will contain the water when we bring the level of the pond up again.
Source: El Yunque Rainforest Bed and Breakfast Blog
Building Your Own Earth Pond (Note: the soil that’s removed can be used to build your earthbag house.)
Erik’s Swimming Pool to Pond Conversion