 68 miners were working more than a kilometre underground when the earth tremor struck, collapsing rock walls around them. More than 40 miners have been brought to the surface and 15 have been taken to hospital. Contact has been lost with two miners. The rescue effort is complicated by the threat of aftershocks and fire risks. Earth tremors are not uncommon in Polish mines, many of which are old and where excavation is taking place deeper and deeper underground.