This is actualy very very difficult to achieve. No you cannot burrow underneath the terrain in OFP. You have to be aware that Metal Gear Solid has a completely different engine, where terrain is defined differently. The only way to make tunnels is to build them into fake mountains, which are hard to make because they need to blend (texture wise) with the actual terrain. Also, its hard to make big complex structutes like that, and not to mention your fake mountain doesn't have terrain properties, it has the properties of a building or static object, so things like footsteps and craters and vehicle tracks won't show, and also, AI will treat it as an object, which adds alot more problems. Not to mention that OFP has bad clipping issues when it comes to these kinds of sctructures, because it simply wasn't designed with compact urban fighting in mind, but the complete opposite, lots of open terrain where you can go pretty much anywhere, thats also why alot of the houses BIS made are not enterable and, if they are, are small and consist of only a few small rooms.