When constructing an indoor pool, we are often asked what get built first the building or pool?
In the cold winter belt, if construction starts in the summer or early fall, contractors frequently erect the natatorium shell with its roof, before the pool is dug. In this scenario, the dirt floor at deck level can serve as a staging area for the construction of the shell and, especially, the roof. Once the building is enclosed, the pool can be constructed and sheltered from the weather during the winter months. In the Sunbelt, the opposite is frequently the case.
When the Goodwill Games was built in Long Island, the pneumatically applied concrete shell was shot against soil as a backform in the summer months. The building was then constructed over the pool with cranes staged outside of the building, which lifted much of the material and components, including the roof trusses. By contrast, the 109’ high roof structure at Georgia Tech was lifted by tower cranes staged on the bottom of the pool after the cast-in-place floor footings and walls were placed. Empty areas of the floor were left for the movement and location of the tower cranes. The floor slab voids were cast after the cranes were removed from the site. 
In the United States, the preference is to avoid below grade tunnels and full basements as a means of reducing construction costs. In Europe and Australia, the custom and practice is to excavate to the foundation footprint of the natatorium at one time, build the cast-in-place pool shell and then finish the other spaces for storage, filtration, switch gear, possible chemical feed systems, storage: chemical storage, etc., plus other storage of dry items for the building.
There are several options to the finished basement or even a perimeter below grade tunnel. At Notre Dame, as well as some other pools, a below grade room was created along one of the 50M walls of the cast-in-place pool, as well as behind the pool wall at one end of the pool. The other two walls were sealed off with concrete walls, but with an access man opening at each end of the space, which has a dirt floor and has pool piping running inside this tunnel. Under this design, access is limited to rare maintenance and repair needs, the space does not require a finished floor, floor drainage, electric lights, or ventilation. All of these can be provided on a temporary basis similar to sewer workers when entering a sewer for maintenance.
The acceptance of this concept is often up to the building code officials and project.