The Ancient Kingdom used Noro-Noro no Mi-like rays to create the Poneglyphs

One Piece Theory

The Noro-Noro no Mi is the secret power of the Ancient Kingdom

What is the Noro-Noro no Mi?

The Noro-Noro no Mi is a Paramecia-type Devil Fruit that grants the ability to shoot rays that slow down whatever they hit. This means that it is incredibly useful for dodging enemies as well as disrupting their movement and timing. While under the effects of the beam, damage accumulates over time, and the effects of this accumulation only occur after time has resumed for the person/object. The rays can also get reflected off of mirrors. Foxy states that the noro particles are an "as-yet undiscovered particle" (although in the story function closer to a wave. Unless the particles/wave are eventually discovered Oda would have left a trail of breadcrumbs unresolved.

The truth of the rays:

The rays interact with normal objects and are a natural phenomenon instead of a magical one (eg. OP-OP no Mi), this leads me to believe that it is not affected by special phenomena such as haki, which makes it one of the best counters to high-level powers within the One Piece world. A reflection of the beams in real life would be radiation. Similarities include the fact that they wear off over time (similar to half-life), only when a certain amount of energy from these rays is on you does the object or person slow down, finally the beams are stated to be like light so perhaps are a part of the light spectrum. Not being discovered yet is a relevant link to radiation as the One Piece world has no evidence of understanding it, in Egghead Vegapunk is seen standing directly in front of the mother flame (essentially a sun) with no protection.

Why are we even talking about the noro-noro no mi over 800 chapters since its last appearance. With what is understood about the fruits powers, it is a perfect candidate to explain the unusual nature of the poneglyphs. They are unable to be damaged, yet are carved and have lasted for at least 900 years. If haki could have destroyed them they would have been destroyed years ago, as they are the only real threat to Imu's reign. So some method unknown to even Vegapunk from 900 years ago was used on the poneglyphs, that points to it being Ancient Kingdom technology. The crux of the theory is that the Ancient kingdom has understanding of radiation and thus noro-noro beams, they then carved the poneglyphs and used the beams with a significantly high half-life to make the poneglyphs indestructible. This concept could be used on other objects such as the ancient weapons and might be what explains how they have lasted so long.

Little add on: Davy back fight was introduced with Foxy which could be foreshadowing the link between him and the past