The lower areas are exhausted and caved-in mine blocks, the higher strips are intact parts between blocks.
Map courtesy of Estonian Land Board, based on LIDAR data. The "brick pattern" is formed by worked out and collapsed mines. On terrain this pattern means around 1-2m differences in height which is hardly noticeable when vegetation is present, but becomes more pronounced on empty fields.
When air is blown out at high speed through the ventilation shafts, it also takes groundwater dripping from the shaft walls with it and together with freezing temperatures outside, the "ice volcanoes" are formed.
When whole blocks collapse, over 1 million tons of rock is displaced by several meters, these mini-earthquakes are registered by seismographs as far as Finland (around 200km away). The longish and low frequency signatures of pillars failing in chain reaction are distinctly different from explosions in nearby quarries.
The houses in the foreground are actually built on caved-in mining area. In the background there are operating oil refineries and a power station, all using oil shale. The mountains are formed from industrial waste.
After oil-shale is worked out and the blocks are not actively used any more, the ceiling support anchors are removed and the roof starts to fall. I was photographing in a nearby chamber during one of these occurences and it was really terrifying experience despite the miners joking about it.
The large connected empty spaces left by closed mines considerably change the groundwater movement, especially when the areas are not flat. Although the holes are drilled and trench is dug, the phenomenon itself is natural as Ahtme mine is situated on the slope of Jõhvi upland.
The mines usually leave the ground beneath houses intact, but this does not apply outbuildings such as barns and warehouses. In this place one of the outbuildings is located on the caved-in mine, while the other is on the border of intact and mined area.
Aerial photos courtesy of Estonian Land Board. Photographs from 1996, 2005, 2009 and 2012 respectively.
This area was plagued by frequent mine collapses and sinkholes due to very shallow mining depth (10-20 meters), so when the new roads were built, additional security measures had to be taken. To protect the roads from collapses, they were built on special reinforced nets.
The large underground bodies of water formed by abandoned and flooded mines would trump many Estonian lakes. Both Tammiku and Ahtme mines are about the volume of fifth largest lake in Estonia - lake Ülemiste from where the capital Tallinn gets it's drinking water.
These pillars are left to carry the weight of 50-70 meters of ground above. Altough most of them hold up for decades, some might still fail and do fail. Due to this uncertainty, the land is defined as quasi-stable and strict limits are imposed on new buildings and infrastructure.
Map courtesy of Estonian Land Board. This map is showing the scale and progress of underground operations. All the worked out area is standing on the pillars as shown in previous picture.
When the ground in already wet area falls by 1-3 meters, it'll form a pond due to water accumulation from nearby areas. Eventually the trees will dry up and fall.
The ventilation shaft blows out humid air and water droplets that form ice on the surrounding trees and bushes, bending them down to the ground.
This place is popular among Estonian cave divers who search for extreme adrenaline and infinite visibility diving in the clear waters of closed and flooded mines.
Most of the oil-shale goes into producing electricity, this in turn creates huge amounts of ash and alkali water that is used to wash the ash out of the boilers.