Belize ... This is called the ATM Tour or ATM Cave. It's amazing, you swim in with headlamps on and climb rocks and ladders to get to different levels. On each 'platform' you observe different artifacts. It's pitch dark inside with flowing water. Once in a lifetime tour, there are 25 guides that go through special training to be allowed to take tourists in the cave.
The busier time of the year for travelers to Belize is November to May, which are the winter months of the Northern Hemisphere. Still, many visitors prefer the more quiet days of the summer months from June to October for their vacation in Belize. Both times of the year have their own unique attractions and the choice of when to visit is totally up to you, your budget and the adventure you crave.