Terrae Cult

A cult outlined in Houses of Hermes: True Lineages, it originally "retained knowledge of huge [Mercurian] rituals that could generate earthquakes and volcanoes", "but lacked the numbers and resources to cast them". Instead,

"they created smaller spells that a single wizard could cast. They had spells that would cause the earth to swallow up their enemies or smash them with a boulder. They developed spells to increase the fertility of earth, giving them a service they could trade for food and shelter... [and] a spell that conjured precious metals from thin air."

The Founder Guernicus was the last of this lineage, but a number of his apprentices "founded a cult dedicated to preserving Guernicus’s earth magic. The cult accepts magi from any House, but the leader is always a Guernicus magus. This leader always finds his way to a seat on the inner council of Magvillus. These magi are extremely fierce in magical combats and often follow the Hoplite career."

Houses of Hermes: True Lineages describes some of the cult's mysteries:

"Amongst these cultists, many possess the divinatory ability of geomantia, making them investigators that even the wiliest fear. Many learn to summon and bargain with earth elementals."

However, the book doesn't give details, saying that "such magic is beyond the scope of this book." This is likely because it was one of the first Fifth Edition books published, and such content was not yet available. The fuller Fifth Edition line can now provide some possible avenues to flesh out the mysteries and magic of the cult:

Geomantia is probably the practice of Geomancy under the Divination mystery descrived in The Mysteries: Revised Edition Chapter Seven: Divination & Augury. Note that Divination is somewhat in competition with Intellego, as investing experience in one comes at the expense of the other for the same (Intellego-effects) benefits.
Summoning and controlling elementals is part of standard Hermetic magic, with new spell guidelines to do so described in Realms of Power: Magic Chapter Eight: Magic Things. Terrae-magi probably developed such spells, as well as spells to imprison elementals.

It is possible the cult retains Mercurian magic, and perhaps even Mercurian rituals related to earth magic, perhaps in the vein of Fenicil Rituals described in Houses of Hermes: True Lineages. Other mysteries are also possible.