The next gust of wind will topple it. If it comes from one direction, the boulder will crash into the rocks, start an avalanche and the villages below will be wiped out; if the wind swings by just a degree or two, the boulder will bounce harmlessly into an empty valley.
Scientists call it the tipping point: the stage where everything hangs in the balance. All it takes is a tiny, nearly random, nudge to trigger events, and the difference between safety and disaster is so small it is impossible to predict which way events will tumble.
That tipping point is exactly where Glasgow Warriors find themselves as they prepare to face Benetton Treviso on Friday. Defeat, which would be their fifth in seven games, and it is odds-on that their season will collapse as they drop out of the RaboDirect Pro12 play-off battle. Victory, particularly if they can score a few tries in the process, and suddenly they can put their Heineken Cup woes behind them and concentrate on making a statement in the league.
The thing is, says lock Tom Ryder, who expects to be back in the running to face the Italians, the problems that have been costing Glasgow their games are themselves tiny. It should not take much for the team to put an end to the slump that started in late October and has resulted in four defeats in six games, with the wins coming against Connacht, last in the league, and the Ospreys' second string on a weekend when Wales were in action.
"You are playing against good teams and the margins are tiny; it is just simple mistakes [that are to blame for the poor results]," said Ryder. "There are plenty of things that have not gone that well, but they are easy to rectify. If they do start going well, we will put teams away; we are playing good rugby and getting into good positions, but it is the last pass or the last little thing is letting us down. Sort that out and we will be back to where we want to be.
"You can't lose four games without feeling that confidence has gone a little bit, but we are an experienced enough team and have enough players who have been through this before to come out of it. What it takes is just one win and everyone is up there and breathing again; three wins and you are top of the league. That is the big thing - these shifts can happen really quickly. That is why this is a massive game for us, a massive point in the season."
Assuming he does play, it is also a massive game for Ryder personally. Having missed the start of the season with the most serious case of concussion he has experienced, he found that others - notably Tim Swinson, capped in the summer, and Jonny Gray, who went on to win caps in November - had moved past him and he has struggled to get game time. Now that his rivals are out for a few weeks, he knows he has to take his chance, just as they took theirs.
"I always say you are only three games from being dropped completely, but also only three games from playing for Scotland. That's just the way it is; you have to work through the low points and make sure they make you stronger," Ryder said. "Second-row is one position where competition is tough, so you have got to perform on the pitch to keep your place with Glasgow and push for honours."