Have you been moving vertexes? Make sure that all of the brushes are still convex 3d shapes. You might have to delete an illegal brush and then replace it. 

That those brushes are forming the hull0 or the boundary between the inside of your level and the void. 

Thanks, I deleted those brushes and made them again (and I also played a little with some points), but I still have leaks, it's driving me crazy, I surrounded the entire map with a big box and I recompiled - I walked around the map where the leak is reported now, but I can't see a leak. It's very, very, VERY annoying, imagine that I have a map that it's working and now it's not working because I changed some walls and the leak it's reported in completly other area :(

I did another thing : I took a copy of the map that is working and I compiled it - everything ok. Then I just added the new tunnel, I didn't delete ANYTHING, just I added the new tunnel over the old tunnel and BAM ! leak - that's f*king impossible :( I hate this aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaarghhhhhhhhhhhhhhhhhhhhhhhhhhhhh 

And I'll take a look. 

Have you been using the WorldCraft carve tool by any chance? 

I use Quark, here's a screenshot :
http://img820.imageshack.us/img820/3828/newtunnel.png

The selected brushed are the only thing that I added (nothing is deleted, for test). The brushes are X mirrored, that's all.

It's beyond my comprehension how ADDING new brushed can make a leak in the map, it's simply impossible, but it happens...

Without the new brushes the map is compiling perfectly. I add some brushes and I have leaks... 

Since I'm not a Quark user, but I suspect its what I said before - the brushes crossing each other at a point that's off-grid, effectively making an off-grid point thats giving you your leak. Maybe. 

is it a leak in hull0, hull1, or hull2? Sometimes hull0 will be healed but one of the other hulls will not because of a compiler error. If this is the case you might be able to plaster over it with a clip brush. 

healed = sealed 

If you can. 'Better :)

Google "quakeadapter" 

ijed, I think you are right, it has to be something off-grid, I think somewhere along a line, it cannot be seen in the map.

I deleted the brushed one by one and I found a brush with problems, I created it again and it's ok, now I have to delete the old brushes from the tunnel, I also delete them one by one, two of them cannot be deleted because the leak appears again, I have to search a little bit, maybe I will find the problem

Thanks everyone for the help, I really appreciate it. 

I oftenly came to the disturbing error of leaks in my map on places that formerly compiled right.

I understand the annoying frustration it gives because mostly the map was right and the compiler gives an unsportive answer.
It took me a lot of maps before I got loos of the boxform while compiling, and always this was caused by the leak error.

Now I know there is no sport in it, it's just so precious adding poly's and making sure that all that's outside the hull must be placed excact on grid.
First I thought forcing on grid on 1 size grid would help, but this works only in boxes and makes the surface looks sloppy.

I think it is just the fault of thinking that the outside of the poly is on grid, while the middle of the poly counts and has to be integer.

The best solution is stop with the leaking map and begin with the one that was good.
If something goes wrong with brushes later on, forcing on grid can help, or adding a sealing brush to the leak pointed ones. 

It works !!!

MadFox, you're right, I make a lot of "snapshots" of my map (I backup often and I keep old maps). Now I just added the walls one by one in the new map, then deleted the old walls one by one - it didn't worked first, then I decided to resize the old walls (moving them inside the old walls) - it worked. Then I tried to delete them again - it still worked, strange, but in the end it's ok.

Now I can go to sleep happy (it's 4:36 AM here) and tommorow I have to be at work at 8:00 AM and I have a very busy day, but who cares ... my map is ok again :) 

It seems surprising to me that a brush intersection that creates an off-grid point would leak the map. Since when does bsp care about the grid? bsp files contain float coordinates, so it should not be a problem. 

it has to do with hull expansion. if a brush intersection causes a problem in hull0, when the collision hulls are created, that minor problem is sometimes magnified such that it basically just gives up on the area, creating those 'holes' where you just fall, seemingly, right out of the map.
aguirre's bsp is pretty good at fixing problems, but some still slip by. other compilers are even touchier. 

Do NOT use the Digger. It is the QuArK equivalent of carving. Do not use it. You will summon everything that sucks about mapping. 

Well, QuArK is a fantastic straight forward editor, very easy to handle... but it suffers of a fucking bad behavior regarding its floating point coordinates capabilities: this makes sometimes the brushes to be off-grid.

The only way to avoid this off-grid corrdinates to be very careful when you move your faces / vertex, and even when positioning the textures on faces.

If you want to correct any leaks or "hole in maps" you have to read this first (The aguirR'es Mapping Holy Bible) http://user.tninet.se/~xir870k/tooltips.txt that will give some very good procedure to solve 99% of your leakage problems.

However, for the remaining 1%, you can still open the .map file generated by QuArK after QBSP process, and look for the floating coordinates: that will give you an indication where are the corrupted brushes, and you'll be able to either replace it, or force it to grid with the editor.

In anyway it is long, it is painful, but it is worth doing it :) 

Spirit, it'a too late for "don't use digger" (for my current map ) - I have tons of diggers there.

But let's say that in the future I will stay away from digger - how can I replace that ? How can I make holes in a brush without using diggers ? 

How can I make holes in a brush without using diggers ?

Easy: you have to make it manually.

Example of a tunnel:
- Bad: you can use two "cubes", inserts a "digger cube" in a "solid cube"...
- Good: you can make it manually with 4 cubes, without digger, just making the hole a real hole with brushes

Dunno if it is clear :P 

For clean mapping you place each brush by hand. A cube room consists of 6 brushes. 

JPG, it's very clear, that's how I do the rooms, tunnels and everything. But I was talking about "real diggers" : imagine that you have a pillar and you want to puch a lot of holes into it.
It's very easy to make a "long" cube, then dig a pyramid in it and multiply that digger.

How do you do that without diggers ? (I know it can be done, but it's hell) 

Sorry - JPL (not JPG) :)
Something like this : http://img841.imageshack.us/img841/4003/holes.png

Imagine that in 3D with a lot more holes (and not only on a column, but with more rows) 

Or make one "section" as a brush, then copy and paste it. This way you have absolute control over what you make. The computer cant screw anything up for you. 

but your pillar sounds kind of like something I've been fiddling with:
http://img833.imageshack.us/img833/5668/zqftest03pillar.jpg

making the whole thing out of chunks that fit to the grid 

A brush at a time is always better. The automatic carving tools just aren't very good. If they were then mapping would be semi-automated, since knowing where to cut stuff needs a good understanding of good build quality that the editor itself doesn't have.

Not that vertex editing and clipping tools aren't good. Like Ricky says, just don't trust the machine to do anything remotely clever.