In The Needs of the Three!, it was explained that Lake Acuity, and indeed all three of Sinnoh's lakes, are the entrances to another world where the lake guardians reside, although they do not actually live in the lakes themselves.
The three were almost immediately confronted by CommanderJupiter, who was easily able to defeat them, despite two of them being Gym Leaders. After the Galactic Bomb was detonated at Lake Valor in Tackling Tangrowth, Uxie emerged from the Acuity Cavern and was quickly captured by Team Galactic.
Lake Acuity appeared in The Battle Rages On and A Novel Test!!, where Mitsumi battled and defeated Jupiter there. However, by bringing up Mitsumi's past as a Team Galactic member, Jupiter was able to throw off Mitsumi and shock Jun, allowing her to take the latter hostage. She then contacted Cyrus, who ordered her to bring Mitsumi to the Team Galactic HQ. Mitsumi refused at first, but Cyrus was able to change her mind, and she reluctantly left with Jupiter, leaving Jun behind in the snow, where he was later found by Hareta.
At each of Sinnoh's lakes, a TM can be found containing a powerful move of a different type. Lake Acuity's TM is the Ice-typeTM14 (Blizzard).
Lake Acuity is the only lake in Sinnoh where Team Galactic isn't fought. Instead, Barry fights Jupiter, losing and vowing to become stronger.