If you’ve ever dived in Lake Pleasant during the summer, you’ve probably experienced it: one minute you’re perfectly warm in your 3mm wetsuit, and the next, you drop just a few feet and hit a wall of icy water that practically takes your breath away. This invisible, freezing barrier is called a thermocline. Here is the science behind why our desert lakes form these dramatic temperature layers and how you can prepare your gear (and your mind) for them on your next local dive.