leaks cannot be "found" when they don't exist, placing blocks is a poor solution, but one that can work if you placed them correctly.

my suggestion, take blocks of sky texture and place them on the back side, overlapping them if necessary to stop up the hole, try to isolate the area, then remove a block at a time till you find and seal all the holes.

you could always put a box on the outside with a light at 10,000 and remove all light from the inside and physically walk in and find them.

you problem is multiple holes, not just one..

Yup, theres leaks at both tunnel ends, doesn't seem to be any leaks anywhere else.

I would redo the tunnel using a 8 unit grid to make it easier to line up the brushes.

Try this for s**t's and giggles...

Right click on the rock formation and "move to world". Then each and every piece will be editable.

I would agree with most of what AS said, can't pick out much else really, some nice curved brushwork and texturing seems ok.

I would give the carve tool a miss, its usually more trouble that its worth.