Zero Event that set of a chain reaction known as the Pattern. Reiden Lake is one of many soft spots on the planet Earth, which are places where the fundamental constants of nature have begun to decay. As a result, in these particular areas, the membrane between universes is thinner. Due to the Zero Event, the barrier between universes was shattered, sending a series of cracks in the very constants of nature rippling throughout the American Northeast. This created a pattern of cracks, soft spots, where science could run haywire. The Pattern is a result of scientific activity being instigated on these unstable soft spots, which can cause unforseen results.
Following the death of Peter Bishop in the Prime Universe, Walter grieved back watching his counterpart attempt to cure the same boy in the Alternate Universe. After Walternate was distracted by September, Walter realized that he would not be able to find a cure in time to save Peter. Walter took it upon himself to cross over and save the boy. The Bishop family was currently vacationing at a lake house near Reiden Lake. Walter constructed his portal on the frozen ice, assuming that the lake would absorb the excess energy from the portal. Nina Sharp and Carla Warren tried to stop Walter from crossing over, fearing that the repercussions could be incredibly dangerous. Walter activated the portal and crossed over. Nina grabbed onto him, but was unable to pull him back before the portal shut, slicing her right arm off in the process. On the Other Side, Walter discovered that the vial containing the cure had shattered in the scuffle. He would have to take Peter back to the Prime Universe and cure him.
Upon Walter's return journey, he discovered that Nina and Carla had left for medical attention. As the two were leaving the lake, the ice cracked, sending both plunging into its depths. They were saved by September, who was attempting to correct his earlier mistake by ensuring that Peter survive, just as he would have if September had not distracted Walternate in the first place.
After curing Peter, Walter realized he could not take Peter home, because of he and his wife's desire to hold onto their son, but also because he realized he had gone against nature and that crossing over again could escalate the damage he had already caused.
Peter, who at first realized he was not home, tried to return to his "home in the lake" by breaking through the ice. He was saved by Elizabeth
Even crossing over once, however, proved to be enough to set both universes off balance. Walter had cracked the very fabric of the universe by crossing through at this delicate soft spot. He accidentally set a chain reaction off, causing a pattern of cracks that would spread across the American Northeast for decades. The cracks would continue to get larger and larger and scientific action caused on these cracks would further weaken them and wreck havoc, known as The Pattern.
ZFT leader David Robert Jones escaped prison in order to cross over to the Alternate Universe and meet with William Bell. He began searching for a place that would enable him to open a portal long enough for him to cross over. Meanwhile, his ZFT agents began committing terrorist attacks that were scientific in nature to further weaken that membrane between the universes, hoping to break down both universes and lead them to a war that would end in their mutual destruction, paving the way for William Bell's Neogenesis plan. Jones attempted to open a portal in several locations, but found that the portal would not stay open long enough for him to safely cross over. He found the right location in Reiden Lake. Hot on his trail, Fringe Division was able to track him down after they discovered his intentions to open a portal. Olivia Dunham realized he was attempting to open portals at locations in the middle of all of the Pattern-related activity. She mapped out the coordinates for every case Fringe Division had investigated and discovered a pattern that led to Reiden Lake. The team was able to track down Jones and Peter closed the portal on him as he was crossing over, slicing Jones in half.
After Peter's death in the Prime Universe and Walternate being distracted by September in the Alternate Universe, Walter crossed over into the Other Universe via portal at Reiden Lake. On his return journey, however, he and Peter fell through the ice. Walter was able to survive, but Peter drowned. This change from the Original Timeline, in which the pair were saved by September, was a result of the Original Timeline Peter using The Vacuum. He realized he was a paradox and created a brand new timeline that could exist without his interference to try and heal both worlds.
The strong connection between Peter and the people he left behind in the Alternate Timeline created a force stronger than science: love. This strong love is believed by September to have been the reason Peter returned to the timeline. Cameron James, a Cortexiphan subject, released massive amounts of energy that provided the impetus to bring Peter back to the timeline in physical form. He appeared in the timeline at the very location he left it: Reiden Lake. He splashed out of the lake completely naked and was rescued by a pair of boaters that were in the lake at the time.