By Adam Easton
BBC News, Warsaw
Safety in Polish coal mines has improved markedly since communist times but accidents still occur regularly.
When communism collapsed in 1989, around 120 miners were killed in Poland that year. Now around 20 die each year in accidents.
Heavy industries like coal mining were the pride of the communist regime. Miners are still paid considerably more than doctors and teachers, but back then the pits were extremely dangerous places to work.
Unions complain that safety measures are not sufficiently tight
The country's worst post-war accident occurred in 1958 when 72 miners died in a fire at the Makoszowy mine. In two separate incidents in the 1970s, explosions killed 34 miners each at mines in Silesia.
"In the communist era, accidents were caused because a priority was placed on production rather than safety. Now that's changed, people understand better," professor Kazimierz Lebecki of Poland's Central Mining Institute told the BBC.
Mr Lebecki said safety had been improved by the introduction of better technologies, a smaller but better trained workforce and reductions in production.
Coal production has decreased by about 50% since communist times, but with current production around 100m metric tons a year, Poland is the European Union's biggest producer.
The last biggest disaster actually happened at the Halemba mine - the scene of the latest accident - in 1990, when 19 miners were killed in a methane gas explosion.
Safety 'not tight'
The Halemba mine in Ruda Slaska, near Katowice, the capital of Poland's coalmining industry, has been in operation for nearly 50 years.
Ventilation conditions are quite poor at the mine and this latest accident occurred at a depth of 1,030m, where methane gas is more highly concentrated.
In February this year there was another methane gas explosion there. One miner was trapped under rubble for 111 hours. He was eventually pulled out alive.
On Tuesday, the miners had gone to retrieve new machinery from the deep mineshaft. The area had been closed in March due to dangerous methane levels. Halemba's management says the work was carried out using specialists in detecting gas. But some trade union leaders believe safety precautions are still not tight enough.
"For the past six or seven years we've been talking to people in the ministry of labour and they've been telling us that safety conditions in the mines are terrible," Solidarity's Dominik Kolorz told TVN24 news channel.
"There maybe more tragedies like this. The frequency of accidents has increased a lot," he added.
It is a claim that is not supported by professor Lebecki.
"Mining authorities and management look after safety more now. Conditions can be bad but, generally, I don't believe that coal mines are like a holiday resort anywhere in the world. Coal mining is one of the most dangerous professions in the world," he said.