mainland vs solid ground

mainland and solid ground both are nouns.

Word	Noun	Adjective	Verb	Adverb
mainland	Yes	No	No	No
solid ground	Yes	No	No	No

As nouns, solid ground is a hypernym of mainland; that is, solid ground is a word with a broader meaning than mainland:

mainland: the main land mass of a country or continent; as distinguished from an island or peninsula
solid ground: the solid part of the earth's surface

Other hypernyms of mainland include dry land, earth, ground, land, terra firma.

mainland (noun)	solid ground (noun)
the main land mass of a country or continent; as distinguished from an island or peninsula	the solid part of the earth's surface