There exists a theory that Louhi, the ruler of the Underworld, is the same as the Earth Mother. This would not be unheard of: the Baltics also believe that the Underworld goddess and earth goddess are the same, and there is a lot of Baltic influence in Finnish mythology. On the other hand, the Sámi equivalents, Máttaráhkká and Jábmiidáhkká, are separate.
While the Underworld can be understood to be in the north, it has also been imagined to be underground. No surprise then that epithets like "girl of the earth" and "maiden of the Underworld" are presented together, and both Louhi and her sisters and daughter get epithets like "girl of the field" and "maiden of crops". Furthermore, Louhi's son is the "son of the field" and "lord of earth". Maatar is also mentioned alongside Syöjätär and is even connected to the creation of snakes much like Underworld figures.
It's clear then, right?
Yeah, about that.
Let's follow this train of thought and see where it will lead us. If Louhi is Maatar, then she is also the "step-mother" who had intercourse with Sämpsä to increase fertility and make plants grow in the spring. As Sämpsä is called the son of the field as well, and this intercourse is implied to be incestuous at least to some level, this could make Sämpsä a son of the Underworld, maybe even the same as the shooter of Väinämöinen.
And this is where it all gets... Not so convincing for me. Louhi and her sons are quite consistently connected to death, frost and cold, not fertility. Louhi kills Lemminkäinen who symbolizes warmth and fertility. Why would she herself, and her son, then be in any way responsible for spring and fertility? It is the sun goddess who resurrects Lemminkäinen from the cold hands of death. For Louhi's daughter fertility connections make more sense as she ended up marrying Lemminkäinen in the end.
Well, there is still one thing going in favour of the theory. Agricola said that either "wife of Rauni-ukko" or "Rauni, wife of Ukko" had fertilizing intercourse with him. Earlier, I have already said that runosongs rather support the idea that the sun goddess has a son with the sky god. However, if we are to assume that Rauni really is the Earth Mother... Well, there is no support to the idea that she'd be Ukko's wife, as the intercourse happens with her and her "stepson". But Louhi does indeed get called Raana, Raani, Rania, Runo, Ranu... such variants. Is it related to Rauni? We can't know for certain, as Raana is similar to words related to used or worthless items or people. Similarly, Louhi is also called Nata, which seems a similar insult on her being a "harlot" rather than a proper noun. This would also make Agricola's sentence "the worthless wife of Ukko", or "the wife of worthless Ukko" which sounds kind of horrible actually. Related "Röönikkä" does still appear in a context where the different names of Ukko are listed.
It could be a matter of both. Shamanistic Underworld goddess on one hand, Baltic inspired earth goddess on the other. The intercourse story in general is considered to have been borrowed from international myths anyways. I could easily hand snakes and stones to Louhi or her daughter, and even accept that she is sometimes called a woman below ground and her son, a son of the field. But I cannot place her in the Sämpsä story, it just does not make sense cosmology-wise. Runosongs also often mention ylisissä taivosissa, alisissa maaemissä "in skies above, in earth mothers below", or ylisillä jumalilla, alisella maaemällä "above with the gods, below with the earth mother". Not only does it speak of plural earth mothers often, it is also difficult to say if it's trying to portray sky-earth or sky-underworld relations.
The Northern Finnish Earth Mother who replaced maahiset as a receiver of milk offerings could also not be Louhi either, though it's not impossible that she is Leävätär. If my theory is true, Leävätär is a wind goddess and a daughter of Louhi. Maatar is the "haltija of treasure pits", but the wind goddess is asked to reveal treasures as the guardian of such a pit and the "subordinate of the old woman of Earth". Old woman of earth and Lady of Earth are equated and put alongside Sämpsä, so they must be the ones intended in incantations of "raising peat". Kaarle Krohn also pointed out that connecting the earth to "Manala", the Underworld, could be a later variation in these epithets: maan -> manuen -> Manulan -> Manalan. One Karelian runosong also implies that a mother that is raised is the singers own dead mother or foremother.