Quick answer: Complete the Yellow Tower key route, then use the finished key at the Yellow Tunnel access around 3500, 1620.

Find the Yellow Tunnel lock

The captured route records the entrance around 3500, 1620, southeast of the Yellow Tower. Descend the mountain on the tower’s south side and follow the rock face. The entrance is opposite one of the blue bag structures; the key slot is on its right side.

Yellow Tunnel entrance with the shaped key near its right-side access slot — The key slot is on the right side of the documented Yellow Tunnel entrance. Screenshot: Screen Hype.

Complete the Yellow Tower first

The Yellow Tower at about 3310, 1560 needs five ordinary rewards. Its captured area contains six documented puzzle options; after the panel releases the key, follow five cutters until the last machine beside the tunnel finishes the shape.

Use landmarks inside the network

The underground route is darker and more enclosed than surface travel. A bag-location walkthrough records one reproducible landmark: a small blue entrance to the left before the giant red staircase, on a route between the Red and Green tunnel sections. That side space contains the purple hip bag.

Prepare the group before entering

Keep at least one light source and one radio pair distributed across different players. Agree which coloured section or staircase is the regroup point before exploring a side passage. Bags help keep tools attached when a player slips, but a teammate is needed to retrieve a tool from the wearer’s slot.

Unlock Big Tunnel

Steam’s visible condition is “Enter a big tunnel.” Entering through the unlocked Yellow entrance satisfies the documented requirement; it does not mention clearing every branch or finding a secret room.

Sources checked

Sources

Captured tunnel route + bag-route cross-check + official achievement · Last fact-check: Aug 11, 2026. See our editorial and corrections policy.

Captured routeScreen Hype — Tunnel route
captured two-player Yellow Tower route and tunnel access
Captured routeScreen Hype — Backpack and hip-bag locations
eleven captured bag locations and carrying setup
Captured routeMonkeyKingHero — Big Tunnel video
visual confirmation of entering the activated large tunnel
OfficialOfficial Steam achievements
achievement names and visible requirements
CommunityBig Walk community tips thread
player field notes for bags, radios, gestures and regrouping