There is one border we don't usually get to linger on; we only ever cross it twice. That is the border between being alive and not being alive. The checkpoints are one way only, and are named Birth and Death.
When we think about it this way, we realise there's no such thing as "being dead". From a materialist point of view, the state before birth and the state after death are the same thing. And yet there's no way, for example, I'd describe myself as having been "dead" in 1960. Yet logically there's no difference whatsoever. Whatever makes you you is your mind, and even though hardcore materialists claim that's all smoke and mirrors, your consciousness has a definite existence however it's derived. So, unconventional theories of time and mind aside, any time period during which your consciousness doesn't exist is no different from another.
So we die, but we are never dead. For instance:
...and so on. Our life span can be seen to be the area of spacetime in which we exist. For instance if we never leave the UK, to a person who never leaves Australia we are the equivalent of dead.
- Where is Douglas Adams?
- He's usually to be found between 1952 and 2001, why?
- So he is dead?
- He isn't anything. The word "is" cannot be applied in his case.
- So where is he?
- The word "is" cannot be applied in his case.
Of course one could counter that example by citing the existence of Skype or telephones, but the principle is the same. Imagine, then, the same two stay-at-home people at opposite ends of a world before these technologies existed. You might then argue that they could still have written letters to each other, but then again so could someone in 1952 write a letter to someone in 2009. In this instance the traffic would be one way, but that's the time dimension of spacetime for you.
Some people send back reports from the final border (usually they're too preoccupied with taking their first breath to pay much attention the first time they cross), these are Near Death Experiences (NDEs) and they're surprisingly consistent. It's not clear whether this is cultural conditioning or not - do people see them selves floating down a tunnel towards a light because they've heard that's what happens or is the tunnel and the light an instrinsic part of the process?
Just what is the process anyway? Bear in mind that these reports are all made by people who didn't actually die (well, duh), so we can never know whether they are also experienced during actual death. Nevertheless, it's a fair assumption, and one can imagine that the whole "life flashing before ones eyes" procedure is the brain searching through the available data for the best response to imminent termination. But where does the tunnel and the meeting of lost loved ones come from?
Some have speculated that it's a scenario manufactured by the brain to make dying easier. I don't buy that at all. How does something like that evolve? Your experiences whilst dying have can have no effect of the surviving gene pool; a positive biological occurrence at this stage cannot possibly be selected for.
If there was a safe way to induce a NDE, I'd jump at it like a shot... there's nothing like first-hand experience to provide insight.
I'd have to be very, very careful though.