A tunnel is a similar to a window in that it allows crossing between the Demonata Universe and the Human Universe. However, a tunnel is far more powerful in that it does not collapse. Furthermore a tunnel once large enough will allow even demon masters to cross to the Human Universe whereas normally only demon familiars are able to. A tunnel also lets a lot more magic flow between realms than a window does.
The spells required to open a tunnel are as long and tedious as those required to seal one.
Sealing a tunnel is just as difficult, if not more difficult than creating one. Mostly due to the fact you have hordes of demons protecting it from being closed.
To seal a tunnel two mages are required, one to chant the spells and another as a sacrifice. Once a tunnel is sealed the mage and demon master who were part of the core are destroyed.