The facility at Buddens Camping and Activity Centre near Wareham is officially opened tomorrow (Saturday) at the Dorset Scouts' annual showcase with a host of activities including archery, football and abseiling.
The Arrowhead Mine has more than 80 metres of tunnels and chambers created from concrete pipes and conduit.
More than 100 offenders working under the Community Payback scheme worked on the project under the supervision of Dorset Probation Area staff.
Dorset Probation Area chief officer John Wiseman said: "This impressive tunnelling complex would not have been built without the labour provided by supervised offenders under Community Payback.
"If the labour was calculated at the rate of minimum wage it would be worth over £25,000."
The Arrowhead Mine and other facilities are available for all youth organisations and schools. Contact centre manager Phil Ventham on 01929 471215 for information.