What the listings say about the walk itself
The defector-day listing is the most specific: about 400 metres at an 11-degree gradient, comfortable clothes and shoes recommended. The small-group and private listings generalize the same reality into a moderate-fitness advisory, and the private day adds poor cardiovascular health and spinal injuries to its not-recommended list. The tunnel is narrow enough that visitors walk stooped in places under a provided helmet, with a monorail-free incline both ways — down is the easy half.
Read those lines as a package with the schedule: the tunnel is a fixed stop with a time slot on every checked route, so "I'll wait at the top" needs the operator's agreement in advance, not an improvisation on the day. If your group includes anyone who will sit the tunnel out, confirm the waiting arrangement when booking.
Prepare the small things
The gradient punishes smooth soles and reward-loop, so wear real walking shoes; bring a layer, because the tunnel stays cool in summer; and leave bags sized for a narrow passage. Photographs inside are restricted to designated points, and the climb back out is the part first-timers underestimate — pace the descent.
For where the tunnel sits in each route's order, see the route comparison; for the identity documents the walk's checkpoint requires, see the passport guide.
Common questions
Is the Third Tunnel walk compulsory on these tours?
It is a scheduled stop on all three checked routes, but sitting it out is a common request. Arrange it with the operator before the day rather than declining at the entrance.
How strenuous is the climb back up?
The published gradient is 11 degrees over roughly 400 metres each way. Healthy adults manage it at a slow pace, but the two listings that publish fitness advisories mean it — read them as written.