It's one of the main draws that brings visitors to Iceland, for a float in its milky-blue, comfortingly warm waters.
But the world-famous Blue Lagoon geothermal pool has closed for a week because of the current seismic activity around the site.
The site is part of southwest Iceland's Reykjanes Peninsula -- a thick finger of land pointing west into the North Atlantic Ocean from capital Reykjavik. As well as the Blue Lagoon, the peninsula is also home to Iceland's main airport, Keflavik International.
Iceland is one of the most active volcanic areas on the planet. Rather than having a central volcano, the Reykjanes Peninsula is dominated by a rift valley, with lava fields and cones.
According to the Icelandic Met Office, around 1,400 earthquakes were measured in the 24 hours leading up to around midday on Thursday November 9, with another 800 in the first 14 hours of Friday. Seven of Thursday's were of a magnitude of four or above -- and all of these were on the peninsula, between Eldvörp, near the airport, and Sýlingarfell, a mountain just to the east of the Blue Lagoon.
The most severe quake registered 4.8 west of Þorbjörn, a mountain roughly a mile south of the Blue Lagoon, at just before 1 a.m. on Thursday.
"It is the largest earthquake since the activity began on October 25th," the Met Office wrote in a bulletin on Thursday.
"While the accumulation of magma continues, seismic activity can be expected on the Reykjavík Peninsula because the magma intrusion causes increased tension in the area."
Magma three miles down
In the previous 24 hours, 1,200 earthquakes were measured, mostly in the same area and at the same depth -- around five kilometers (three miles) below ground level. "It is likely that seismic activity will continue, and be episodic in intensity, while magma accumulation is ongoing," the wrote on November 8, adding that "uplift continues in the area."
By Friday, the area was still active, with around 800 quakes already measured since midnight, according to a 2 p.m. local time bulletin from the Icelandic Met Office. A "dense swarm" of quakes started at 7 a.m., culminating in a 4.1 magnitude quake near Sýlingarfell shortly before 2 p.m..
However it was also at pains to point out that an eruption isn't necessarily imminent. "The fact that there are now larger earthquakes than before in the area does not necessarily mean an increased rate of magma accumulation," said Thursday's Met Office bulletin.
Friday's bulletin warned that earthquakes of up to 5.5 magnitude "can be expected" as magma accumulation continues below ground. However, they noted, "At this stage, there are no indications that magma is forcing its way to the surface."
An explosive recent past
The Reykjanes Peninsula has now been coded yellow -- one above the rest of the country's green -- for eruption risk.
According to a statement from Visit Reykjanes, the local tourist board, the current activity is similar to that preceding last year's eruption of Fagradalsfjall, around 8.5 miles southwest of the Blue Lagoon. Fagradalsfjall has erupted every year since 2021. The latest eruption took place from July 10 to August 8, 2023. As hiking trails were reopened at that time, visitors were warned not to walk on the still "steaming hot" lava.
The Blue Lagoon's pool, spa, hotels and restaurants closed on November 9 and are scheduled to reopen at 7 a.m. on November 16, according to a statement on the website, which continues: "The primary reason for taking these precautionary measures is our unwavering commitment to safety and wellbeing."
Any visitors with bookings will receive a full refund.
Other tour operators near Mount Þorbjörn have also suspended activities until at least November 16, according to the local tourist board. One road has been closed, and access to the Elvörp geosite has been halted.
The closures will be reevaluated on November 16, according to a notice from the tourist board.
According to the BBC, a representative from the Icelandic Civil Protection Agency has been sent to Grindavik, the town 2.5 miles south of the Blue Lagoon, to "prepare for a potential evacuation of the town" in the event of "magma appearing to ruse to the surface."
RÚV, the Icelandic National Broadcasting Service, has set up a livestream of the area, including the Blue Lagoon.
CNN has contacted the Blue Lagoon, the Icelandic Met Office and the Icelandic Civil Protection Agency for comment.
