Lag and Underground

[Kinetic plasma]

I have a world where I cleared a large space underground and blocked the games day light and put buildings underground with sun blocks. I also have buildings on the surface above the underground buildings. Ever since I added the underground part to my map I've been getting a lot more a bad lag spikes. Is my game lagging because of the underground part of my map? If s, is it possible I can reduce the lag spikes by filling underground back with grass and pretend like I never messed with clearing space underground? How effective would it be? Note this is happening to me on the xbox360, not the PC.

[Jaz]

The more block faces that are able to be seen, the more stress you're putting on the game as it needs to render such. It is recommended to fill up any spaces where players will not see and have no useless block faces showing, if your area under the map is as big as I imagine then filling it should help in some form - This was a big issue in the 360 version of the game.