In the Editor Environment attributes window, in the Fog section, there is a slider for fog altitude (base level). This slider's range includes negative values (below sea level). That works fine, the negative values are fully supported and quite useful. However, the negative values can only be used by moving the slider. If you try typing a negative value into the associated text field, it will automatically change to the same number but positive. This is awkward if you want a specific negative value, since trying to get exact numbers by clicking on the slider is almost impossible.