You've traveled back in time 65 million years, with no way to return. What evidence can you leave to ensure future humans will know of your existence?
This is very dependent on the technology you have available.
Given enough tech my approach would be retroreflectors on the moon. Build in as much redundancy as you can, both in the number of reflectors and how each is set up.
Each reflector has a gold wire coming off it leading to a buried capsule containing your messsage etched in metal foil, layered with a different metal so the whole thing doesn't just weld together.
If I couldn't get to the moon my second choice would be to head to Namibia and hope the desert there already exists.
If so I would go to a point deep in the desert and bury several message capsules. I would then make a ring around this many miles out. Drill a deep hole, place in it a tube. On the bottom is a lump of U-235, on the top is another lump. There is a support holding that upper lump that has radioactive material incorporated into it such that the decay slowly weakens the support. Calibrate these to break in 64 million years. (Of course this won't be perfect, the actual failures will occur over time. If you can be precise enough edge it closer to 65 million years) If the tube is undamaged when the support breaks there is a decent chance you get a nuclear detonation.
(BTW, for those who think you need something more complex: This is not a reliable bomb, you have a decent chance of a fizzle. It also must be U-235, a Pu-239 bomb like this is a certain fizzle, not to mention that your Pu-239 bomb will have long since decayed. However, if even one explodes it will be noted and fizzles will be found.)
In time somebody is going to discover the remains of your bombs. While natural nuclear reactors exist there's no known way to get a natural nuclear detonation. Thus even the discovery of one bomb crater (remember, the craters aren't all that old, they'll be much more visible than anything you did 65 million years ago) proves intelligence at work and would trigger a major effort to figure out what's up. If you get three detonations you provide an indication of the location of your buried messages.
This was originally posted to the Puzzling SE, which means that the poster has a specific answer in mind.
Since they chose the odd value "65 million years" instead of a round number, it is probable that the intended answer is:
You should cause the Cretaceous–Paleogene extinction event.
The problem with this question is you don't say what I have available to use to send a message. If I have no modern tech, I'm probably dead before I can leave anything that lasts a few years, let alone millions. Bacteria, animals way more experienced in eating other animals, unknown flora, lack of safe food supplies... I'm in too much trouble to worry about sending a message, I'm just trying to survive the night.
If, however, I'm a fully equipped traveller from the future with food and medicine enough to last a while, I would probably do what that one answer on the dupe question suggested and smelt gold and silver into coins and scatter them all over the North American Craton in hopes one might be found. Of course, Gold and Silver are nice for chemical stability, but not very sturdy for mechanical stress, so if I was able to smelt steel I might do the same with steel roundels and ingots. I'd have to find some nice, boggy place to put my stuff, and hope somebody finds it... I'm kinda lucky, because humans have all of future history to find it.
The best way to send a message forward from 65m years ago? If I have the ability to travel back in time, with any luck I've got the technology to easily achieve orbit. Using my advanced computer from the future, I'd compute a trajectory that would put a microsatellite in the vicinity of Earth in 65 million years, and hope I have the delta-V to put it there.
Or, if I have the Delta-V, I'd try to put a metal or stone tablet at a Lagrange point; then I'd hope that somebody in the future would put a probe or park a ship there and wonder what that artifact is floating nearby. With any luck, the solar wind wouldn't have destroyed my message.
The key, though, is that 65 million years is a long time.
The real question is would anything be left after that long? Even metal alloys would erode incredibly in that time. Even so if you made them unmistakably intelligent shapes, it still could be un-recognizable by then. There is a chance if you made enough of the same shapes, and scattered them around the globe there would be evidence left. Personally I like the Mt. Rushmore effect. So my answer is spend your entire life, aside from time spent surviving, carving and breaking the biggest rock formation on earth, and then spend even more time burying the formation in sand and dirt. If you made a sign or symbol in the rock that was large enough, maybe even carved out of a mountain, it might just stay. especially if you buried it hoping that humans would detect it later with some sort of sonar or infrared device, we use those devices nowadays to detect oil and mineral deposits.
Forget metals. Gold aside, none will last 65M years. Instead, think fired ceramics.. A humble dinner-plate will survive until it is ground into dust by erosion or melted along with the rock it has become part of. It's a perfect techno-fossil.
You can easily mass-produce a message for the future using a mould or a stamp on your wet clay. Aim for quantity not quality, because you have no good way to predict where to leave your message tablets so they will survive and be near the surface 65M years hence.
That's it, really. Scatter them around and hope. We can be pretty certain there was no house-brick using dinosaur civilisation because no such fossil bricks have been found
BTW if you have the technology to make big hydrogen bombs, and are willing to unleash such on the environment, evidence of that should survive. Craters lined with trinitite and shock quartz, and various long-lived isotopes that are not found in nature. The fallout layer might be global and detectable, as was the Iridium from the vaporised meteor that killed the dinosaurs. But would you survive that fallout? And, you'd still need ceramic tablets to carry an actual message.
Just few things that can survive that long, and most of them are hard to do and require process like many had answered here.
But you can do something that's quite easy, not a hard thing to do (while it need something to be hard), and it's already imprinted on our DNA :
just breed. Fill the Earth with your offsprings.
And then tell your saga to your kids. For sure it would change the history of earth and mankind, but you get your message to the future anyway.
In light of the feathered dinosaur tail that was found embedded in amber, I suggest you go hunting, then write a note, pin it to the dino, cover it in tree sap, and bury it.
Do this a bunch of times and wait.
You could also just write a bunch of notes and cover them in tree sap without the dinosaurs, but that's less fun.
Being from the present, you could know what artifacts survived through the years from memories of looking through museums and literature in your proper time. Also you would know the location of famous fossil digs, cave paintings etc.
This would enable you to pick sites with the most potential of being discovered. Lest your meddling interfere with history, definitely make many messages, but at least you know where and what has the best chances of being found, as you've seen it happen already.
Assume your traveler bumped in the year 1999 (65m years before his original time).
What he can do to ensure his identity maintains for years and people of his time get to know his identity?
Be Like Nostradamus!
The traveler can sensationally opt to tell and share famous good or bad internationally known events which will occur after few years of 1999 BC. People of 1999 may not believe him until they see your first story coming true. Also the word "Sensationally" is important. People ought to remember things which are said in unique or unconventional ways.
For example: He tells people of 1999 about 9/11/2001 attack in NY something like this:
*Five and forty degrees, the sky shall burn:
To the great new city shall the fire draw nigh.
With vehemence the flames shall spread and churn
When with the Normans they conclusions try.*
(Source of these lines)
And in 2001 when the incident happens, you can start your marketing and publicity in full swing (indirectly via your devotees or friends of course). People will record and publish your predictions (stories) and will continue to pass on the same if the incidents he chose to tell actually kept on occuring in consecutive times.
I am sure, a future citizen of 65m years away may not even know about something that happened on 9/11/2001 but I am sure that few events stays in history textbooks for thousands of years. Take old scriptures and religious transcripts.
Your traveler can even inscribe your stories in monuments and such buildings which are mostly restricted or protected and have potential to stay for years. Even if these monuments gets damaged, their photos, transcripts and literature will be kept for future people to show them how their "ancestors" way of architecting was artistic (like Big ben or The Taj Mahal)
And if not textbooks or monuments, use people and generations to pass on your stories (and most importantly your identity which at some point of time will reach the right year to target person).
I bet metals and monuments may not stay for such a long time but folklore and literature does