The main drain is the bottom suction typically located in the deep end often two drains will be tee-d together with just the one pipe returning to the equipment
While it seems obvios to many of us I wanted to point out that many people think the main drain is just that a drain. they may be looking for a valve. I guess the name is a little confusing. in worrking on thousands of pools i have seen one actual drain it was on a city pool. for the rest of us a "main drain" is just a "skimmer" on the bottom it improves circulation and allows you to run your pool without the skimmers if the need should arise.(looking for leak ?) and can be used to pump out pool for liner replacement or plater/tile repair.
I would add to that. The main drain, which is not a drain in the literal sense, like in household Plumbing, but another means for better water circulation. Without a main drain and just the use of a skimmer or skimmers only the upper two feet of water in the pool would circulate leaving the lower two thirds of the pool water to be somewhat stagnate. This lack of circulation, sometimes seen as sand or dust or bugs are seen to collect in one or two patches of the floor, can and will cause reduced sanitizer in those areas resulting in algae in or near those areas. Also, there are no valves for the main drain in most newer pool construction. Fifteen plus years ago some builders used valves for many functions on the pool equipment pad to redirect water flow and there was a valve specifically for the main drain. The gate valve was mainly for the pool maintenance tech to help in vac the pool -- by closing that valve he could vac the pool faster because the closed valve directed all the flow to the skimmer.
Vacuum the pool. If you have to drain it, only drain it half way. Floating pools are a disaster.
Why would you drain your pool just becasue the water is hot? Never drain your inground pool unless you have first cut holes in the bottom of the pool to keep it from floating. If the water table in your area is high enough, your inground pool could "float" out of the ground, effectively ruining your pool.
You don't drain an inground swimming pool, unless you are trained to do so. Doing it alone or even with assistance is extremely dangerous. Save yourself some trouble and go contact a local and trained swimming pool cleaning service.
To remove excess rain water in an inground pool, you likely want to get a filter. You can also install a drain at the very bottom of the pool, but this will need to be closed if there were water in the pool for swimming.
I have seen them at intheswim.com.
The skimmer is the healthiest method for removing water from a pool for filtration at all times
Justin Bieber has a inground pool
sounds like there is ground water getting in there somehow
If they feel main drains are not required that brings their marketing/ sales practices into question.
You should install a small pump 1/6 horse lets say, let it run.
They don't make them for in ground.