Jules Verne hypothesized, 

"Anything one man can imagine, other men can make real."  

and observed no doubt,

"Reality provides us with facts so romantic that imagination itself could add nothing to them." 

and to offer a final quote from this French author, accidental to our purpose perhaps.

 "With time and thought one can do a good job."  

Einstein wrote, as a result of his mathematical calculations that an object whilst not actually doing so, would appear to the naked eye to zigzag very fast on the spot just before it  transported to another time.  

I have seen this very phenomenon. A spaceship doing just that. It was the classic "cigar" type.

Time holes exist on the earth in various places and at various times where some of the  earths energy lines intersect. Amazing things happen as a result - beyond our control and understanding. A different time will emerge in that place with the physical features associated with that time and none of the present, unless there has been no noticeable change to the environment in that  time.

This to date is anecdotal and not tested in the field by scientific research.** This may happen provided, we hope, the intentions of the scientific research are good and honourable, therefore perhaps best not to occur at this particular time in our history!

Now we come, if we accept the possibility at least, of time travel,  to a startling conclusion. There is no past, no future anymore in the sense that we have become accustomed to understanding it. There is only a continuum similar to the continuum of space and we can traverse it similarly, with the right technology if we want to. No doubt many may not.

Modern technology is an exponential curve over a say 200 year period say, of development.

Who is to say this 200 year curve and more, much more, has not arisen, before recorded history, ie. existent records which  survived the ravages of man and time?

It is easy to imagine this possibility. Indeed there is nothing to contradict it and we have physical evidence still surviving of a more advanced technology than our own.

Because of this 200 year exponential development of technology in our own time we can even hypothesise, indeed we must, that a longer time curve of technological development could easily have occurred in a past epoch, lost to us through natural and/or man-made catastrophe(s), (there is no need to assume just one), of worldwide breadth and depth. 

Therefore it is not unfeasible or even unreasonable that a development way beyond our own level of technology has taken place in the past of which we have some surviving glimpses.

The question is: how far did this development go and did it include time travel?

One might be tempted to suggest that if they developed to such an advanced level it seems highly unlikely that they would not have had the technology to prevent such man made or even natural catastrophes.

But is not the record of our own technological development coincidental also with our own environmental destruction? Yes. So this hypothesis can be dismissed out of hand.

But we may surmise, if I may entertain the possibility, that their technology allowed them to predict a coming catastrophe.

And this is where time travel, if it exists, may have played an important part in attempts to counter the destruction. We know that they failed.

What we do not know is

 1. the size of the worldwide population which was destroyed - and the evidence shows that it was worldwide.

 2. the lifespan of the individuals 

 3. to what extent they deployed time travel, if at all, and for what period of time before the catastrophe(s) occurred.

From the latter we have a very good, hypothetical at least, explanation for the current rash of flying saucer, ufo sightings and this explanation is NOT at variance with the level of technology exhibited even by the fleeting remains of a past civilization(s) which still exist, scattered across the globe, including at the bottom of the sea, much of which we must assume also, remains to be discovered.