Is vertical forced movement into the ground legal, or not?

Draw Steel: Heroes, p. 271:
>If a forced movement effect has the word “vertical” in front of it, then the forced movement can move a target up or down in addition to horizontally. For example, if a forced movement effect says “vertical push 5,” then a creature targeted by the effect can be pushed up to 5 squares in any direction, as long as the forced movement is a straight line.

>If a creature who can’t fly is left in midair at the end of a vertical forced move, they fall. Forced movement made against a creature who is flying is always a vertical forced move, whether or not the effect specifies it.

>Though you can’t freely push, pull, or slide a target up and down unless that forced movement specifies “vertical,” you can move them along a physical slope such as a hill or staircase. For a target to be force moved along a slope, each square of the slope can be no more than 1 square higher or lower than the previous square.

Same book, glossary:
>Vertical: When any form of forced moved is noted as vertical, the creature performing the forced movement can move the target up or down (though not into the ground) in addition to horizontally. p. 271