My robot sinks into the ground

[omichel]

How can I fix this?

[omichel]

If your robot is heavy and sinks into the ground constantly

Your robot weighs over hundreds of kilograms.

1. Reduce the basicTimeStep field value of the WorldInfo node.
2. Reduce the softCFM field of the ContactProperties node which defines the contact between the robot (typically the robot wheels or feet) and the ground.

If your robot suddenly sinks into the ground (ElevationGrid) as it moves from one tile to another

This is an ODE bug. A workaround is to make the ElevationGrid tiles larger than the size of the robot.