The Nahuku lava tube in Hawai‘i Volcanoes National Park that is frequented by hikers is closed until further notice after movement was detected in the rock over the weekend.

A fracture near the top of the lava tube was found to have narrowed by 2.47 millimeters, which could indicate that other fractures are widening, possibly making a large overhead rock “unstable and potentially dangerous,” the National Park Service said in a news release Monday.

Data from a crack meter in the lava tube moved 1.96 millimeters Saturday and another 0.51 millimeters by Monday, according to NPS officials. The park closed the tube Sunday, without a clear reopening date.

Eric Bilderback, a geomorphologist with the National Park Service, alerted management to the movement on Sunday. He called the movement “slow and not accelerating” but “unusual in the monitoring record,” according to the news release.

Until last weekend, data showed less than one millimeter of movement in the tube since November 2019, the NPS said. A similar pattern of movement would need to resume before officials consider the tube safe to reopen.

After the 2018 eruption and summit collapse of Kilauea, large rocks were dislodged in the lava tube, and two crack meters were installed in the lava tube’s ceiling to monitor movement thereafter, officials said.

The restrooms near Nahuku and the parking area are still open.