Dolmen

The Secret Grove cannot be accessed until you beat the Ferronightmare boss. After beating this boss, you will see a cutscene where two doors become accessible (the light up with green lights). One of these doorways leads to the Secret Grove.

You can get to the Secret Grove from the boss chamber, but I'll detail how to get there from a teleporter.

From the Power Plant teleporter, go down the hallway and head out into the walkway areas leading to the various rooms/buildings. Descend the first set of ramps and when you come to a crossroads, turn left. Turning right will take you to the boss chamber, and straight ahead leads to a room with green and red tubes.

After turning left, head down the stairs, go through the doorway (it'll open when you approach), and go through another doorway on the far right-side wall. Once through that doorway, take the elevator down, and go forward into another building. After entering that building, take the open doorway in the right wall; you'll be entering a rocky area.

After a few turns, you'll see the announcement that you've entered the Secret Grove.