The Cave was the last location in the southeast corner of the map. It has been closed off for decades, but the energy inside is still strong. It forced Fisica to use the Purify function on the Notalium Tuner because of how strong the energy was, when the function was still in development.
The Cave seems to have a great connection to the rest of the Notalium in the world because, after tuning the Notalium in the Cave, the Notalium around the world awoke in "resonance," and their supply of energy gradually grew in strength.
After the Notalium in the Cave is tuned, the Notalium Administration begins to collect Notalium from it to use as energy for the town.