How comes a free-diver does not get crushed by the pressure when diving?
Every 10 meters under water, the pressure increases by 1 bar, which is approximately one standard atmosphere unit.
This means for example at 50 meters deep, the pressure the diver is receiving is six times higher than at the surface.
For most of the body, composed of tissues and fluids, this is not a problem as they are incompressible.
However, the air contained in the lungs being pressurised will reduce in volume inversely proportionally to the pressure. 
If the individual dives with a tank, he will be able to inflate his lungs, but if the diver is free-diving, the volume of air is limited (being the one taken at the surface).
In our previous example, the volume contained in the lungs would be divided by 6, which is in principle below their residual volume.
This means lungs will not be able to deform themselves anymore, and a depression will appear in the thorax of the free-diver
hence risking the lungs to collapse.
The latter would though not happen, as the depression created in the lungs from not being able to get deformed anymore will generate an influx of blood that strengthen the lungs structure, protecting them from being damaged.