Following an 18-day closure, the National Park Service has reopened the Nahuku lava tube in Hawaii Volcanoes National Park. Rock in the lava tube was found to have been moving more than usual, but the movement now appears to have returned to normal, according to the agency’s news release.

“Conditions are constantly changing on this living lava landscape, but safety is always our top priority,” Volcanoes Superintendent Rhonda Loh said in the news release. Officials collected data showing the movement decreased to normal levels and inspected the tube itself. Even so, the Park Service warned tube-goers of the risks: falling rocks, low ceilings, tripping hazards, standing water, and low or no light.

During the 2018 eruption of Kilauea, large rocks came loose in Nahuku, prompting officials to install two crackmeters to monitor movement in the lava tube. The hollow was formerly known as the Thurston lava tube.