The Tunnel Club and the fridge (p. 672)
So the unspoken consensus is that a hamster’d be optimal but they’d settle for a rat. Just one lousy rat could give the whole Club a legit raison, an explicable reason for congregating underground—all of them are a bit uneasy about liking to congregate underground for no good or clear reason.
Context
The Tunnel Club's charter is famously vague, and here the scene says out loud what the whole expedition is for: the boys already hold the territory — they're drawn under the school by pure fetish — and what they lack is the map's other half, a legend that would justify the holding. One confirmed rodent would retroactively make the territory mean something. The territory precedes its map, and the mapless occupation makes them uneasy.