Look at the passage (p. 9-12) about searching the tunnels. How is “the waiting worse than the tunnel itself”?

In essence, we learn that the soldiers waiting outside of the tunnel were overcome by worry and anticipation. Anything could happen in the tunnel.... and it did. Waiting for a comrade, unaware of what they'd found or if they would return, was worse than going into it, because the person searching didn't have time to think.