What terrain feature can severely affect ground wave transmission?

Dense forests can severely affect ground wave transmission due to the significant absorption and scattering of radio waves that occur within their complex structure. The trees and underbrush can impede the transmission of radio signals, which rely on a clear line of sight and minimal obstructions to propagate effectively. The materials composing the trees, including leaves and branches, absorb and reflect the radio waves, which can lead to a loss of signal strength and range. In contrast, flat landscapes, coastal regions, and agricultural fields typically offer less disruption, allowing for better ground wave transmission. This is because they tend to have fewer obstructions and less absorption of the radio frequencies compared to dense forested areas.