On September 21, 2011, lava in the west lava lake in Puu Ōō Crater fed a series of lava flows that traveled down the west flank of Puu Ōō during September 20–21. At about 0225 UTC on September 21, activity in the crater and overflows to the west suddenly decreased, as lava broke through the upper east flank of Puu Ōō, bypassing the crater. The new fissure fed a channelized aā lava flow that advanced rapidly downslope southeast. A second flow to the west of the first began the next day. In addition, a small pad of lava actively refilled the bottom of the drained east lava lake and small flows were barely active at the west edge of Puu Ōō Crater. The channelized aā lava flow reached long on 23 September and then stalled within the Kahauale'a Natural Area Reserve. Most of the active lava spread south and west of Pu'u Halulu ( northeast of Puu Ōō) during 23–27 September. Minor lava activity resumed within Pu'u 'O'o Crater with short lava flows issuing from the base of the east wall on 25 September and from the west wall base during 25–26 September. The crater floor of Puu Ōō slowly subsided. Lava activity resumed within the east lake on 26 September. The floor of the crater continued to subside during 26–27 September, opening up cracks in the north crater floor.