The wet cell batterys that most people use gas so they need to be kept in a ventilated space. The gas is flamable.
AGM (absorbed glass mat) batterys can be mounted anywhere. They are sealed & do not vent gas at all. They are commonly used in cruising boats because batterys are sometimes located in places other than the engine compartment. They are maintenance free. No water to top up. They can be mounted any way you please. Even upside down. They give 2-3 times the lifespan of wet cell batterys. They are not cheap like wet cells but all of the advantages more than compensate for the added expense for some.