I know what the black goo really is.

this where it gets complicated. on one hand the substance inside the ampules, when released, kills all the engineers on contact but leaves plant life intact which makes sense. the engineers wanted to kill what ever species inhabited the planet but not leave the planet damaged. so why does the mote, if it is indeed the same substance/ organic organism, that came out the ampules not just kill the 2 crew members and instead react in a completely different manner?

because the plant caused it to mutate to use it as a defense mechanism, and how better to defend ones self than prevent any potential further harm by basically creating an army

u may be right but id suggest its been an oversight by the engineers when designing the substance. I think the basic purpose of the weapon was simply to wipe out the indigenous species and not interact and affect plant life but when the spores r disturbed it looks more like a fungus than plant and fungi are neither classed as plant life or organic so the engineers may have over looked the effect on fungi. so in a way u could very well be onto something, its just that its fungi that causes the mutation to the substance.

It just does when and what Ridley wants it to do. Had they taken the bombing scene out would have been really bad, still love it no matter how confusing it is.

Hopefully we will never get the total answer about what it is. There are some things that I wish that they will keep a secret about. Now when we have some answers to the Xeno I wish that they will keep the origin of the goo a secret.

WAIT I JUST CAME BACK TO THE SITE FOR THE FIRST TIME IN A LONG TIME, AND I JUST REALIZED, I PREDICTED THE BLACK GOO BEING A FOR OF AI THAT BREAKS DOWN THE ORGANISM, now I am kinda worried what else I predicted, and now I am scared for the future of the franchise because if I who at the time was 17 could predict what would happen am worried about if it will begin to become repetitive, ... or it could just be because I like to think that I am really good at putting pieces together...

I think the Black Goo ended up being some kind of McGuffin

They seem to chop and change what it does, my theory just as Prometheus came out was that it is a Creation Tool for breaking down a Organisms DNA/Genomes and becoming a Substance that contains and passes the Sacrificed Organisms DNA onto what ever Organic Life it came into contact with.

Which lead me to the Black Goo in the Urns being the collected results of a Sacrificed Deacon Organism (what ever was in the Mural)  a few months after this conclusion we saw Jon Spaights Alien Engineers leaked and his Nano-Scarabs basically did EXACTLY what i saw the Goo in Prometheus doing.

FOX released that Weyland File that basically tried to explain the Black Goo but contradicted what we saw in Prometheus.

Prior to Alien Covenant and seeing the Bombardment in The Crossing it made me look at another theory i had to explain why the Set Photos showed what looked like Burnt/Mummified Engineers, i thought the Urns were like some kind of Weapon like a Nuclear Device.... where the Ampoules contained one component of the Weapon like how a Nuclear Weapon has say Uranium/Plutonium and how contact with Uranium/Plutonium does not burn you to a crisp like the Blast of a Atomic Bomb, but it causes Radiation that leads to Mutations and other Complications.

So the Ampoules and maybe the Slime Stuff around it act like the Uranium/Plutonium and so on LV-223 what we saw was like walking into a Nuclear Weapons Facility at seeing a leak of Radiation from the Uranium/Plutonium

What we saw in Alien Covenant was the Primed and Activated Effects of the Urns, where something happens to them to cause a Reaction that causes them to Explode, much like how a Nuclear Bomb uses the Uranium/Plutonium

The other theory i had prior to watching AC, was maybe the other Ships Cargo that David took worked differently to the ones in Prometheus.

After seeing Alien Covenant, i was lead to indeed my Nuke Theory where some part of the Weapon Activation maybe via different frequencies of Sound/Radiation something, causes different effects/actions to the Bio-Weapon.   The Black Goo did explode into like tiny particles that seemed to indeed hunt down the Engineers (maybe this is not the case who knows) but it appeared to enter the Engineers and then begin to replicate itself, and escape... but then for some reason the Engineers Bodies just Turned Solid, as if Genetic Building Blocks instead of breaking down, where being Cystalized

I was wondering why this happened..... a Programed way for the Bio-Weapon determined by its Activation/Priming?

The effect of the Black Goo maybe hitting and breaking through some kind of Force Field that protected the City?

Prior to AC i put some theories on here wondering what the Device on the Cathedral Dome was, and could it have been a Safety Device Activated after the Bombardment that Halted the Pathogen, Turning all Organic Material within a Radius into like Stone (Solid) so as to Halt the Black Goo effects. At the time i thought that would explain the Gold Device, and those Bodies.

After Alien Covenant i still considered this... and the Nuke Analogy (Life outside the Blast Radius, suffers the effects of Radiation/Mutations) 

But then we had the Extras to Alien Covenant, were David confirms the Black Goo is like a Radical Form of AI