Radar from space · December 2020 – November 2025
The salt mountain under Praid
Under Praid, in Harghita County, stands a mountain of salt that reaches about 2.7 km down. In May 2025 a swollen stream broke into its mine and filled it with water. European radar satellites measure how the ground moves, a few millimetres at a time. Over Praid they saw nothing outside the ordinary. You can check the ground under your own street.
Five years of radar over Romanian ground#
The Copernicus ground motion service reads Sentinel-1 radar images taken on 303 time-series dates between December 2020 and November 2025. For every 100 m square where the radar finds something solid, such as roofs, roads or bare rock, it gives the speed at which the ground rose or fell. 1.9 million squares have a usable measurement. They cover about 8% of the country, mostly towns, roads and bare rock. Fields and forests mostly give nothing.
Blue means up, grey means no measurable change, amber to red means down. In 94 of 100 built-up squares the ground moved by less than 2 mm a year. About 1 in 180 sank by 5 mm a year or more. The median built-up square moved −0.4 mm a year, so small negative values are normal everywhere.
Praid on the radar map#
Here is the terrain around the mine, with the radar squares on top. Inside the dashed circle, 1 km from the mine, the radar follows 153 squares. In the ring 4 to 6 km away it follows 407.
Before the flood, the median square inside the circle moved at −0.85 mm a year and the median square in the ring at −0.38. That gap, −0.47 mm a year, did not meet the thresholds set for the test further down.
The mountain is mostly underground#
Salt is lighter than the rock around it. Over millions of years it creeps upward and pierces the layers above, like a frozen fountain. Praid stands on the top of one such dome. Published descriptions give it an oval plan of about 1.2 by 1.4 km and a root about 2.7 km down. Its salt layers stand almost upright and are folded.
Horizontal and vertical scale are the same, with no stretching, except for the village’s houses and the mine rooms, which are enlarged so they can be seen. The surface line is sampled from the Copernicus terrain model, and the coloured strip above it is the radar. The dome and the rock layers are drawn, not surveyed, and the dome’s shape below the mine is unknown.
A mine inside the salt#
Visitors, reportedly about 400,000 a year before the flood, walked in a gallery roughly 120 m under the village. The working floors go deeper, to around 400 m. No verified detailed mine plan was obtained, so rooms and shafts here are a schematic of seven floors, drawn taller than life.
Fresh water can enlarge openings as it dissolves salt.
2023 – May 2024
A stream beside the mountain#
The Corund stream runs along the salt. Infiltrations from it into the mine were reported from 2023 and got worse from April 2024. In May 2024 the operator, Salrom, closed the mine for about a month. A long-term repair project was put off to 2025.
5–9 May 2025
The mine closes#
The mine closed on 5 May 2025 after new infiltrations from the Corund. A state of alert was decided on 7 May and ran from 8 May. By 9 May the water authority had begun emergency works with pumps and a membrane.
27–28 May 2025
The stream swells#
On 27 May the flow of the Corund reached about 60 cubic metres a second, and reports say it damaged the membrane. The next day crews began two concrete dams underground, one in the Telegdy sector and one on the way to the tourist gallery. The dams in the picture are drawn.
29 May 2025, about 10:52
The dam gives way#
Salrom reported that a dam gave way at about 10:52. The Telegdy sector, it said, could no longer be saved from the water. In the picture both dams break; the sources name the dams, not which failed first.
4 June 2025
About 4 million cubic metres#
The head of the emergency service, Raed Arafat, said the mine was completely flooded, with about 4 million cubic metres of water. For scale, that is roughly 1,600 Olympic swimming pools of 2,500 cubic metres each. The water level in the picture is schematic.
Above ground, and downstream#
On 1 June, 27 people were moved out preventively. By 2 June the prefecture was ordering the ground levelled where it had collapsed near the old mine. The state of alert has been extended month after month; the latest extension found, from the Harghita prefecture, runs from 6 November to 5 December 2026 and keeps access to the area closed. Officials watch the ground with drone laser scans, seismic stations and a total station. In late July 2026 the seismic stations detected microvibrations, while drone and total-station observations showed no change in relief. In early August 2026 reports described the crater extending.
The Corund flows into the Târnava Mică. From June 2025, salty water cut the tap water of about 40,000 people in Târnăveni and nearby communes. Supply was cut for 12 days in Târnăveni; by September the tap water was still not drinkable.
What the satellites saw#
This comparison and its thresholds were written down before its results were calculated; a simpler Praid analysis had been seen earlier for another page. Would the radar show a signal over Praid before the flood, and a change after it, that stands out from 30 other places across Romania? Neither condition was met.
Before. Ground within 1 km of the mine sank 0.47 mm a year more than ground 4 to 6 km away, against a set minimum of 1 mm. In 19 of the 30 other places the gap was smaller in size; in 4 it was more negative, so Praid ranks fifth most negative of 31.
After. June to November 2025 differed by +0.14 mm from the same months of 2021–2024: the ground inside rose slightly against the ring. The set minimum was 1 mm, about 7 times larger. Ordinary years vary by about 0.09 mm, and this change was 1.6 times that, short of the 2 required. In 14 of the 30 other places the change was smaller.
The sharp dip in the Praid line, 28 May to 15 June 2023, reverses within three weeks and comes two years before the flood. A reversible dip like this is typical of radar noise, not ground motion; it was not investigated further. Praid’s line uses all its squares and the others at most 150 each, so Praid’s is smoother than the rest by construction.
This does not show that the ground is still at Praid. The radar only measures squares where it finds something solid, and the collapsed ground may lie where it has no square. The series ends on 25 November 2025. A literature search found no separate published study of ground motion at Praid after the flood.
Is the ground rising or sinking under your street?
Pick a town or village, then a street. The answer uses the radar squares within 100 m of the street, from the same data as the story. Zoom the map with + and − and tap a square to read it.
Try:
How this was made
Radar data
European Ground Motion Service (EGMS), Ortho product, vertical component, "2025 release": 100 m squares, 9 December 2020 to 25 November 2025, 303 time-series dates. Read through the open EGMS viewer. The newest release listed by Copernicus on 4 November 2026 covers the same period. Generated using European Union’s Copernicus Land Monitoring Service information (realizat folosind informații ale Serviciului Copernicus de monitorizare a suprafeței terestre al Uniunii Europene).
The test
Written before any number was computed: pre-registration (SHA-256 eab3a53e53d5b7e8…). It compares the ground within 1 km of the mine with ground 4–6 km away, before the flood and in June to November 2025 against the same months of 2021–2024, and sets Praid against 30 randomly drawn places in Romania. Praid passes only if both gaps exceed the 95th percentile of those places and a fixed minimum. A first, simpler test of the same place had been run for another page, and its summary was read before this one was written. Praid is compared with its own ring; the 30 places show how large such gaps are elsewhere. Praid uses every square, the other places a random 150 at most.
What the radar cannot do
It sees only squares that hold a solid, stable reflector. Open fields and forest give no square. Each square averages points inside 100 m. The numbers are relative to GPS stations, so a shared offset of the whole region can remain. Streets here are matched to squares within 100 m, from OpenStreetMap street lines, and a street with few squares is only partly measured.
Drawn, not measured
The salt dome, rock layers, mine floors, dams, water level, flood path and collapses in the section are drawn from published descriptions and reports. Only the surface line (Copernicus GLO-30 terrain model) and the radar strip are measured. A paper on the flood by one author at the University of Miskolc was retracted in 2025 and is not cited here.
Sources
- Salt dome size and depth (oval, about 1.2 by 1.4 km, about 2.7 km high) and mine levels to about 400 m: Gelencsér et al., Acta Geodaetica et Geophysica 2024 (citing Horváth 2019); showcaves.com, Praid salt mine. Independent confirmation was not found, so the depth is medium confidence, and it is the dome’s vertical extent, not a surveyed root.
- Upright, folded salt layers: Dohan, Tamás, Tamás and Tocariu, EGU General Assembly 2024, abstract EGU24-685.
- Flood chronology: Radio Free Europe Romania, “Salina Praid inundată” and the four-months report; Salrom statement of 29 May 2025 (Tion, Mediafax); Raed Arafat on Digi24, 4 June 2025; Harghita prefecture notices on the alert extensions (latest read 13 November 2026); Jurnalul and ProTV, July 2026, on the monitoring. The alert was decided on 7 May and ran from 8 May; outlets differ by a day.
- Târnăveni water: Radio Free Europe Romania, four months after (about 40,000 people; 12 days without supply); G4Media, 12 September 2025.
- Visitors before the flood, about 400,000 a year: Harghita prefect, quoted by G4Media.
- Terrain surface model: Copernicus DEM GLO-30, © DLR e.V. 2010–2014 and © Airbus Defence and Space GmbH 2014–2018, provided under COPERNICUS by the European Union and ESA. Streets and places: OpenStreetMap contributors (ODbL), Romanian extract of 24 October 2026.