Objective: To evaluate the effect of pulmonary rehabilitation and climatic therapy conducted in underground salt chambers on the clinical condition of patients with stable COPD. Research Question: Does pulmonary rehabilitation conducted in a subterranean salt mine environment offer measurable clinical benefits compared to standard surface-based rehabilitation? Study Design and

Methods: This is a randomized, controlled, four-arm interventional study including 80 adult patients with a confirmed diagnosis of COPD according to the GOLD 2025 criteria. Participants will be randomly assigned to one of four groups: Group A: Pulmonary rehabilitation in underground therapeutic salt chambers (physical training + microclimate exposure). Group B: Pulmonary rehabilitation in a surface-level gymnasium (physical training without microclimate exposure). Group C: Passive stay in underground salt chambers (microclimate exposure without physical training). Group D: Health education only (control group, surface level). The intervention program consists of 16 consecutive working days of 3-hour daily sessions. The content of the rehabilitation varies depending on the assigned group and includes general conditioning exercises, respiratory exercises, relaxation training, and health education. Outcome Measures and Assessment Tools: Clinical assessments will be conducted at three time points: prior to the intervention (T0), immediately after the intervention (T1), and at a 3-month follow-up (T2 - for Groups A and B only). The monitored variables include: Exercise Tolerance: Incremental Shuttle Walk Test (ISWT) Health-Related Quality of Life: St. George's Respiratory Questionnaire COPD Symptoms and Risk Indexes: COPD Assessment Test (CAT), BODE Index Muscle Strength: Deadlift test, hand dynamometer, 30-second Chair Stand Test Respiratory Muscle Strength: Maximal Inspiratory Pressure (MIP) and Maximal Expiratory Pressure (MEP) Body Composition: Bioelectrical Impedance Analysis (BIA) Dyspnea Scales: Borg Scale, modified Medical Research Council (mMRC) scale Chest Mobility: Measuring tape assessment The study is designed to evaluate both the immediate and long-term effects of different pulmonary rehabilitation models on clinical outcomes in COPD patients, with a focus on whether the underground environment enhances rehabilitation efficacy.