This location can be reached by going south through Deep Woods, or north from Outskirts of Prehevil - Broken Shack - either by drawing a Vinushka sigil in the shack's ritual circle to climb the cliff early, or by lowering the ladder after exiting the Deeper Woods first from the south.
There is a narrow space between the trees directly north of the Owl cultist that leads to the Deepest Woods, which is where you can find the entrance to Tunnel 1. Also, there is a more concealed passage near the leaf patches leading to the Hidden Spot, found by going west of the leaves, then directly south-west into the trees.
If you have the Undergrowth Awareness skill, you can access a secret exit leading to the Maiden Forest in the east of Deeper Woods - there is a short path leading to a dead end, but you can interact with the trees on your right to push through them. Note that this is a one way exit and you won't be able to return this way.
Olivia: "Hee, sorry if I'm a like child in a candy store."
Olivia: "I'm just in my element here. I could spend forever studying the beds of moss, the foliage, the forage, every tree from top to the bottom roots and the mycelium that grows underneath..."
Karin: "It's unsettling to say the least. I feel like there is something constantly moving in the woods! Just slightly out of our view..."
Olivia: "Oh..."
Olivia: "I'm sure it's nothing..."
Olivia: "Just a gentle breeze moving the leaves!"
Karin: "..."
Karin: "I feel no breeze..."
Otherwise:
Karin: "Are you guys feeling the same? Maybe it's just paranoia, but I feel like there is something constantly moving in the woods... Just slightly out of our view..."
Levi: "I wasn't a big fan when growing up... But after spending the majority of my boot camp in a forest and a good amount of my service hiding in one..."
Levi: "It's become a second home to me... Or maybe the only home really..."