More than £154m has been awarded by the government to fund stabilisation works on Bath's historic Combe Down mines.
Work on the mines is expected to be complete by 2010
The mines are home to a colony of rare horseshoe bats and are also a serious hazard to 1,500 people and their homes.
A school and three churches also rest on the thin crust of ground above the deep underground cavities.
The work includes 10km of underground "safe routes" to protect the land and properties above, and long-term bat routes, roosting and breeding sites.
The £154.6 million grant will be paid by English Partnerships - the Government's regeneration agency - over a period of five years.
Planning Minister Keith Hill said: "I am delighted that people and their homes will be protected in the long term, and that this important part of our heritage will be saved for future generations to enjoy."
The proposed work on controlled infill will be delivered through Bath and North East Somerset Council's Combe Down Stone Mines Project Team.
The total amount includes the existing £22.7 million which has been used for on-going emergency work.
The main works are expected to start in late 2005 and be completed in 2010.