archipelago vs solid ground

archipelago and solid ground both are nouns.

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

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

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

archipelago (noun)	solid ground (noun)
a group of many islands in a large body of water	the solid part of the earth's surface