Just looked up the passage in which he first gets into the pensieve cause I was curious:

The tip of his nose touched the strange substance into which he was staring. Dumbledore’s office gave an almighty lurch - Harry was thrown forwards and pitched headfirst into the substance inside the basin - But his head did not hit the stone bottom. He was falling through something icy cold and black; it was like being sucked into a dark whirlpool - And suddenly, he found hinself sitting on a bench at the end of the room inside the basin, a bench raised high above the others