A smelly carpet and underlay can be caused by a variety of factors, including:

1. Moisture: If your carpet and underlay become wet and are not properly dried, they can develop a musty odour caused by mould and mildew growth. This can be especially problematic in humid environments or areas prone to water damage.
2. Pet urine and faeces: If you have pets, their urine and faeces can soak into the carpet and underlay, causing a foul odour. This can be difficult to remove, especially if the urine has had time to soak in and dry.
3. Spills and stains: Spills and stains that are not properly cleaned can lead to a foul odour over time. For example, spilled milk or food can attract bacteria and cause a bad smell.
4. Poor ventilation: A lack of proper ventilation can lead to a build-up of moisture and pollutants in the air, which can contribute to a smelly carpet and underlay.
5. Smoking: If someone in the home smokes, the smoke can settle into the carpet and underlay, causing a persistent odour.
6. Old age: Over time, carpets and underlays can become old and worn out, leading to a build-up of dirt, dust, and other particles that can cause a bad smell.

To eliminate a smelly carpet and underlay, it is important to identify and address the underlying cause. This may involve deep cleaning, drying out moisture, addressing pet stains and odours, improving ventilation, or replacing old and worn-out materials. Professional carpet cleaning services can be effective in removing persistent doors and restoring the freshness of your carpet and underlay.