It's actually two lumps of bogwood, similar size, placed upright against each other which has formed a cave inside and compliments the dimensions of the tank. I bought several pieces when I was looking at putting some in there so I could play around with those I felt worked the best.
The fish love swimming in and out of it, the pleco lives up inside it and the shrimp crawl over it all day long. Until I emptied the tank to change the substrate I had no idea I still had all four of my Amano shrimp in there.