What is strange about the woods and what effect did they have on the cows in Tuck Everlasting?

The wood is otherworldly like it is asking to be left alone.

There is something strange about the woods outside Winnie Foster’s house.  The woods seem to be telling people to leave them alone.  Is there magic there?

But the wood had a sleeping, otherworld appearance that made you want to speak in whispers. This, at least, is what the cows must have thought: "Let it keep its peace; we won't disturb it." (Ch. 1) 

In fact, there is magic in these woods.  The woods hide a special spring, and if a person drinks from this spring he or she turns immortal.  The spring has not been found by many people, presumably. The Fosters, who own the wood, are not even aware of it.  The Tucks drank from it by accident one day, and now they will live forever.

We are told that the cows are the reason no one found the spring, because the cows made their road around the wood instead of going through the wood.  Everyone else followed the cows, so no one ever found the spring.

The people would have noticed the giant ash tree at the center of the wood, and then, in time, they'd have noticed the little spring bubbling up among its roots in spite of the pebbles piled there to conceal it. And that would have been a disaster so immense that this weary old earth, owned or not to its fiery core, would have trembled on its axis like a beetle on a pin. (Ch. 1) 

The Tucks were traveling through the wood when they drank from the spring.  It was not until later when they did not die after accidents that they realized what had happened.  The horse even drank, and it could not be killed.  The cat didn’t drink, and it lived a natural life.