The reason the Underground became a bomb shelter in the first place is because Londoners demanded it. Once the bombings began, people began sheltering overnight in Tube stations by just buying rail tickets and refusing to leave, according to the U.K. government agency Transport for London.

In response, the government began regulating the Underground as an official nighttime shelter. By the end of September, over a hundred thousand people were sleeping overnight in the Underground. Shelterers had to clear out in the morning so that normal rail service could resume. Some even woke up and took the train to work.

Photos from these shelters show pretty rough sleeping arrangements. Some Londoners made their makeshift beds in the actual railway tubes that trains normally ran through. Others slept in train cars, on escalators or on the floor of the platforms themselves. It wasn’t comfortable, but many people likely felt it was safer than sleeping in an aboveground bed.

Still, the London Underground wasn’t impervious to attacks. Blitz bombings killed 68 people sheltering at the Balham station on October 14, 1940, and 111 people at the Bank station on January 11, 1941.

As the bombings continued that autumn, government officials and volunteers began providing services to the people sheltering in the London Underground. These included providing food and water, installing lavatories and building triple-bunk beds in some stations.

Sanitation and health was a major issue for Underground shelters. Medical workers began setting up first aid posts to tend to those taking shelter there. Still, the spread of infectious diseases in the poorly-ventilated rail stations was a significant problem.

Edith Summerskill, a doctor and member of Parliament, drew attention to this problem in the Manchester Evening News. “Already a complaint known as ‘shelter throat’ is common,” she wrote in October 1940, “but the danger to children forced to sleep under conditions which provide a breeding place for influenza, pneumonia, and tuberculosis, quite apart from the more common infectious diseases, cannot be over-emphasised.”

Underground stations became sites of recreation and entertainment during the Blitz, helping to boost morale in a desperate situation. Musicians and acrobats performed for audiences in crowded Tube stations. In addition, shelterers played records on gramophones, exchanged books and held parties.

The last major attack of the Blitz bombing campaign was on May 11, 1941. After this, most people who’d been sleeping in the Underground went back to sleeping aboveground, but some returned to the Underground to seek shelter at different points during the war. After the Allies’ successful D-Day operation in June 1944, Germany launched an attack on London that sent some residents back to the Underground, Mackay writes.

Even though the U.K. government did not want to use the Underground as a shelter when the Blitz started, the Underground’s stations became a popular haven during the bombing. The shelters provided relative safety as well as community and entertainment during a bombing campaign that killed over 40,000 civilians in the U.K.