G. Brindha writes about this amazing cave temple in the Hindu
One has to descend 82 steps to reach the underground tunnels. One has to squeeze in, crawl and worm one's way into the tunnel, holding on to the supportive chains on the walls and taking care not to slip. Between the stairways there is a footlinga of Lord Narasimha. At this point, one can see both ways up to the entrance and down to the main cave. On reaching down, one sees a big idol of Seshnag, holding the Earth on his hood.
There is a Hawan Kund in front. It is said that King Janamejayan performed Nagayaga here, as instructed by Rishi Ullanga to avenge the death of his father King Parikshit. From thereon, one scrambles on the spine of Seshnag to reach the point where water trickles from an eight-petal lotus on the head of Ganesha.
You can read about the wonders found in Patal Bhuvaneshwar Cave Temple in Uttarakhand here in The Hindu