Amundsen marked depots with crosswise flag rows to catch lost parties

On the Antarctic plateau there are no landmarks: no trees, hills or roads to triangulate against, only white in every direction, frequently erased further by fog and blowing snow. Amundsen's plan for reaching the South Pole depended on a chain of supply depots laid down in advance, each stocked with the food and fuel a five-man party would need on both the outward run and the return — miss one, on a return leg with dwindling rations, and the miss could be fatal.

The obvious way to mark a depot is to put a flag on it, or a line of flags along the route leading to it. But a single point is easy to walk past unseen in poor visibility, and a line of markers running parallel to the direction of travel fails exactly when it matters most: if a sledging party drifts even slightly off its compass bearing in a whiteout, it can pass within a few hundred metres of the depot's own line of flags and see nothing, with no way to know which side of the line it has missed on.

Laid numbered flagged poles in a row running east-west across the line of travel, so any party in the wide corridor crossed it and read the numbers to find the depot.

Perpendicular rows intersect the lateral drift of a lost party, so they cross the markers even if off course.

Numbered poles let a party that crosses the line compute direction and distance to the depot.

This design against likely error ensured every depot was found on both legs of the 1,860-mile trip.

When a single marker can be missed, spread redundant signals across the axis where searchers actually drift, not just along the intended path.

Amundsen's party located every depot it laid on both the outward and return legs of the roughly 1,860-mile round trip to the Pole and back in 1911–1912, reaching Framheim without a single resupply failure. He recorded the method himself, explaining that markers running along the route "may easily be missed in fog" while a cross-line "cannot be missed" if the party is anywhere near its intended course.