A landfill is a carefully designed structure for the disposal of waste, where various layers are used to manage the waste and control pollution. Among these layers, it is crucial to prevent the movement of leachate—a liquid that percolates through the waste and can contain harmful substances. Let's explore the options provided to determine which layer prevents leachate migration.

1. Soil Cover: Typically used to cover the waste, the soil cover helps in reducing odor, preventing pest infestations, and managing rainwater infiltration. It does not primarily stop the movement of leachate.
2. Gas Vent: Installed in landfills to release gases generated by decomposing waste, such as methane, but they do not play a role in preventing leachate migration.
3. Impermeable Liner: This is a synthetic layer placed at the bottom of a landfill. Its primary function is to act as a barrier to protect the underlying soil and groundwater from contamination by leachate. These liners are often made from materials like clay or synthetic polymers, designed to be impermeable.
4. Waste Layer: Simply consists of the waste materials deposited in the landfill. This layer contributes to leachate generation but does not prevent its movement.

Thus, the correct answer is the Impermeable Liner. This layer is specifically engineered to prevent leachate from migrating out of the landfill, thereby protecting the environment.

To conclude, the Impermeable Liner is an essential component in landfill design, providing a crucial barrier against leachate migration.