How do you make an under water base in gmod without it filling up

you cant unless you know something i dont


actually it is possible to make an underwater base but not in gmod. you can use the hammer editor to make a map then add water to it and take out the water in the parts of the map where you dont want it. if you have css you can map it with that and i thing you can also map it with tf2. there might be other source games which you can map it with but if there are i don't know about them. and once your done making the map you can export it the which ever game you used then play it on gmod. hope this helps!