Today every city has a water supply and a sewage system, transporting fresh water into any house, and used water from houses and factories to the purification plant. In less environment friendly countries, the sewage is sometimes conducted into the next river or the sea. And this system of water tubes is typically, you guessed it: underground.
Many cities have sewage systems which are big enough to walk through. If a city is big enough, there is need for such big tubes, to transport the sewage water, at least for the main lines. During heavy rains the amount of sewage water may rise a hundred times, because of rain water collected by the sewage system. And it is easier to maintain the tubes, if it is possible to walk inside and remove jams or repair damages.
Of course, the inside of a sewage system does not smell like Chanel No 5. And so it is very unlikely to become a tourist attraction. So it is quite impressive how many sewer system actually are famous tourist attractions.