A 60-year-old World War II pillbox is being turned into a retreat for the declining bat population.
It's a tight squeeze for volunteers but the entrance will be blocked up
Volunteers are working in the cramped, cold and damp building at Sudbury in Suffolk to convert it into an ideal hibernating hideaway.
They are blocking almost every hole and providing plenty of hanging space.
Peter Ennis, from the Dedham Vale and Stour Valley Project, said they were hoping to attract Pipistrelle, Daubenton's and Brown long-eared bats.
All three species have been recorded feeding over the River Stour and nearby pastures during the summer months.
Mr Ennis said once it was ready, the concrete pillbox, which has walls three feet (91cm) thick, would not be a sheltering spot for large numbers of bats.
"It is not going to be your image of David Attenborough with millions of bats pouring out - it doesn't work like that in Britain. It'll be five - 10 maybe," he said.
One of the last and most important jobs for the volunteers will be to seal up the entrance with a steel door.
During hibernation bats require constant air temperature and high humidity so body heat or the heat from a torch could ruin the environment for bats, whose numbers have been in decline for 60 years.
There are plans to convert 10 World War II pillboxes into hibernation sites for bats in Essex and Suffolk.