Switch back to Edge select (cube with line icon).
Select the four edges of the square again (hold Shift and Alt) in addition to the bottom you just created.
Navigate to the Mesh menu > Edges > Bridge Edge Loops.
An "edge loop" is a loop of edges around a model, all connected in some way. In this case, we're connecting the loop consisting of the outside of the terrain with the loop that makes up the outside of the bottom, essentially "bridging" the gap between the edge loops.
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