In sewers, various gases can be found due to the decomposition of organic matter and the presence of bacteria. The most commonly found gases are hydrogen sulphide (H2S), carbon dioxide (CO2), and methane (CH4). Let's discuss each of these gases in detail:

1. Hydrogen sulphide (H2S):
- Hydrogen sulphide is a colorless and highly toxic gas with the characteristic smell of rotten eggs.
- It is produced during the breakdown of organic matter, such as sewage, by anaerobic bacteria.
- Sewers provide an ideal environment for the growth of these bacteria, resulting in the release of hydrogen sulphide gas.
- Exposure to high levels of hydrogen sulphide can cause respiratory problems, eye irritation, and even death in extreme cases.

2. Carbon dioxide (CO2):
- Carbon dioxide is a naturally occurring gas that is produced during the process of cellular respiration.
- In sewers, carbon dioxide is released by the bacteria that decompose organic matter.
- The concentration of carbon dioxide in sewers can increase in poorly ventilated areas, posing a risk to workers or individuals entering such spaces.
- High levels of carbon dioxide can cause dizziness, headaches, and difficulty breathing.

3. Methane (CH4):
- Methane is a flammable and odorless gas that is produced by the anaerobic digestion of organic matter.
- It is a byproduct of the bacterial decomposition of sewage in sewers.
- Methane is lighter than air and can accumulate in confined spaces, increasing the risk of explosion or suffocation.
- In addition to being a potential safety hazard, methane is also a potent greenhouse gas.

Therefore, in sewers, all of these three gases, hydrogen sulphide, carbon dioxide, and methane, can be found. It is important to take precautions and ensure proper ventilation when working in or entering sewer systems to minimize the risks associated with these gases.