Nothing. i don't know if you can see properly from the front shot, becoz it's very difficult to portray depth via photography.
but if you see in real life, there are actually 3 valleys. and since i cannot capture it on film, here's a badly drawn top down representation showing the three valleys.
the valleys are not going to be filled with anything and will remain empty. but the rocks flaking the valleys will have corals bah. the whole idea is to have empty space with sandbed.
coz there's nothing i like more than looking at sand in a reef tank. alot of people don't realise, but having big patches of visible sand spaces greatly enhances the spaciousness of the tank. '
unfortunately, the sandbed is brown now.