Many people also believe in the Easter Bunny and that the earth is flat...
If they are from ancient civilizations beneath the planet (how does that work exactly, anyway?), then they would not be "aliens".
As for being from other dimensions, what does that imply? That their location in their dimension is in close proximity to our location in ours? That would be quite a coincidence.
Titus' assumptions are much like my own when it comes to aliens.
Assumption #1: There IS other intelligent life out there, but the universe is incomprehensibly large. With hundreds of billions of galaxies, each containing hundreds of billions of stars, it is very likely that other intelligent life exists elsewhere in the universe. It would be more remarkable if it did not.
Assumption #2: Aliens CAN travel at the speed of light. I don't believe this, but I am accepting it in theory.
Given these two assumptions, what can we deduce? Mankind has been producing signals (radio, TV, etc.) for about 130 years (first radio transmission was in 1897). That means that in order for an alien civilization to even be able to detect intelligent life from their location, they would have to be within 130 light-years of Earth (incredibly unlikely, given the size of the universe).
IF they were within that distance, and IF they could travel at the speed of light, and IF they left their location the moment they detected our presence, they STILL wouldn't be able to get here for another 130 years from NOW.
Here's a question: today, almost everyone in any civilized country on the planet has a smart phone capable of taking very good, high resolution photos and videos, and they have them with them at all times. Yet somehow, no one has managed to get a single decent photo or video of an alien craft. Why? The only ones I've seen are blurry, jumpy, or easily identifiable as something like an airplane. I love how eye witnesses (in numerous "sightings") describe the "alien" ship as being triangular, with flashing red and green lights. Hmmm, what else has flashing red and green lights? Oh, I know! AN AIRPLANE! And with a flashing green light on one wingtip, a flashing red light on the other, and a white one on the rear, what shape do they form? A triangle.
There is no Easter Bunny, the Earth is round, and aliens have NEVER been here. They aren't flying around observing us, they aren't mutilating livestock, and they aren't abducting people and shoving probes up their butts.
People are stupid, and people are gullible. The "aliens have been here" people are right up there with the "we never landed on the moon" clowns.
For all of them, I have this (well, this and a boatload of disdain):
