The denser cluster of northbound thru-hikers who embark from Springer Mountain the last week of March and the first week of April. Although much smaller, the cluster of southbound thru-hikers who embark from Mount Katahdin the first week of June is also sometimes referred to as a bubble.
Example: “I left on March 10th to avoid hiking with the Bubble.”
Fun Fact: The most common start date for AT thru-hikers is usually April 1st, although March 1st, March 15th, the first day of spring, and St. Patrick’s Day are also common. To protect and preserve the land near Springer Mountain, the ATC recommends that thru-hikers attempt to vary their start dates.