Within the spell, it is stated that:
You make terrain in an area up to 1 mile square look, sound, smell, and even feel like some other sort of terrain. The terrain's general shape remains the same, however. Open fields or a road could be made to resemble a swamp, hill, crevasse, or some other difficult or impassable terrain. A pond can be made to seem like a grassy meadow, a precipice like a gentle slope, or a rock-strewn gully like a wide and smooth road.
Similarly, you can alter the appearance of structures, or add them where none are present. The spell doesn't disguise, conceal, or add creatures.
The illusion includes audible, visual, tactile, and olfactory elements, so it can turn clear ground into difficult terrain (or vice versa) or otherwise impede movement through the area. Any piece of the illusory terrain (such as a rock or stick) that is removed from the spell's area disappears immediately.
Creatures with truesight can see through the illusion to the terrain's true form; however, all other elements of the illusion remain, so while the creature is aware of the illusion's presence, the creature can still physically interact with the illusion.
Furthermore, per Crawford, you can harm someone with the effects of the illusion:
The mirage arcane spell gives you tremendous latitude in how you make
the affected terrain look and feel. The altered terrain can even hurt
someone. You could drown in the spell’s illusory lake, for example, or
fall off an illusory cliff.
For all intents and purposes, the spell appears to be able to give you blanket authority on what the terrain will be within a 1 mile square area.
Does this authority extend above the ground at all and if so, how high?
The Crawford tweet seems to suggest a degree of ability to affect the terrain vertically by virtue of drowning someone within an illusory lake and the spell's description seems to suggest that I can turn solid ground into something with inconvenient depth, like a swamp; plus I can add buildings within the terrain if I so choose without any stated limit.
For example, were I in an underwater adventure, could the party Wizard cast the spell upon the sea floor and remove the water at the bottom of the ocean and make it more habitable for the rest of the party (and perhaps less suitable for the BBEG)?