How to create a floating rock?

This question is inspired from the Indian Sanskrit epic Ramayana, in which Lord Ram builds a bridge Rama Setu connecting India and Sri Lanka, which was entirely made of floating rocks.

Two Engineers made this marvel possible who can make the rocks float. I would like to know if anything like this is possible to achieve given access to high-tech alien engineering skills.

Requirements are something like this:

  • These things may or may not be actual rocks.
  • They should float.
  • These floating rocks should look and feel like rocks to a normal person (so, styrofoam is not allowed).
  • They can have varying sizes (palm size rocks to huge boulders).
  • When floating they should be able to withstand the ocean waves.
  • They should be able to bear the weight of Rama's entire army crossing the bridge.

How to construct the bridge itself is not of interest right now.

  • 13
    $\begingroup$ Pumice - it would be one word comment. Especially if you don't care about "how to". $\endgroup$ Commented Jun 2, 2020 at 9:14
  • 2
    $\begingroup$ @AlexP, Of course, it does not float, otherwise, we will be awestruck, But in Ramayana, it floated. That story is the inspiration of this question, not the limestone shoals. $\endgroup$
    – V.Aggarwal
    Commented Jun 2, 2020 at 9:32
  • 8
    $\begingroup$ @V.Aggarwal Of course it would. The important thing, that you don't want to know, is construction. You show you made zero research because even wikipedia link to pumice raft. $\endgroup$ Commented Jun 2, 2020 at 9:44
  • 2
    $\begingroup$ @V.Aggarwal Pumice can have a density as low as 0.25 g/cm^3 -- if ice with a density of 0.92 g/cm^3 can support a army, a properly constructed pumice bridge could too. $\endgroup$
    – Johnny
    Commented Jun 2, 2020 at 21:10
  • 3
    $\begingroup$ "Sheldon fun fact!" The Gordian knot was made of the ropes that held together the pontoon bridge that allowed the Persian army to invade Greece ala Thermopylae/Marathon/Salamis. The Greeks rolled it up and put it in the temple of Gordos as a war trophy. Legend said whoever could untie it would be the leader of all Greece. Alexander the Great hacked it to pieces with his sword. $\endgroup$
    – DWKraus
    Commented Jun 2, 2020 at 23:01

7 Answers 7


Concrete pontoons

I'd give you a link but since it's a standard product they're all commercial sites.

  • They float.
  • They're rock-like, you can sculpt what you like into the top or just stick a layer of actual rock on top.
  • Load capacity in multiple tons, they'll take an army.

You could make a few gravelly bits to float around them, you could make them irregular shapes so they're not so obviously constructed objects, but for the sake of a legend concrete pontoons will happily tick all the boxes.

  • 2
    $\begingroup$ Exactly what I was going to say... anything (watertight) floats if its mass to displacement ratio is low enough. Geodes might also work if you need something more "natural". $\endgroup$
    – Matthew
    Commented Jun 2, 2020 at 14:33
  • 1
    $\begingroup$ Concrete ships were fairly common, and used in WWI and WWII: en.wikipedia.org/wiki/Concrete_ship Concrete boats are a thing among hobbyists: my university had (and may still have) a team that entered an annual concrete canoe competition. $\endgroup$
    – jamesqf
    Commented Jun 2, 2020 at 16:23
  • $\begingroup$ And you don't really need high-tech aliens. We've done this in the Netherlands since 1953, when the sea tried if it could win from us. The English build them even earlier, for war preparations to invade Normandy in 1944. $\endgroup$
    – Mast
    Commented Jun 3, 2020 at 9:47
  • $\begingroup$ @jamesqf Technically I think they're "ferrocement" because they don't include coarse gravel in the mix, but the misnomer is long-standing and isn't likely to go away. :) $\endgroup$
    – Graham
    Commented Jun 3, 2020 at 12:55

Pumice rafts are already thing.

Typically created by volcanic eruptions, they can be gigantic -- one off New Zealand in August 2012 was reportedly 480 kilometres (300 mi) long and about 50 kilometres (30 mi) wide, with pumice blocks poking up to 60 centimetres (2 ft) above the ocean surface.

This shows that, will a little effort (e.g. using rope or other fiber to hold the raft together, and matting to provide a firm surface) it should be possible to create a pontoon bridge using pumice for the floats.

  • 3
    $\begingroup$ "Raft" is a highly misleading term here. A "pumice raft" is simply a mass of pumice that's been temporarily bunched up by wave or wind action. It's about as good for crossing the water as a bridge made of soap bubbles. $\endgroup$
    – Mark
    Commented Jun 2, 2020 at 23:26
  • 1
    $\begingroup$ @Mark, yes, but the question is about constructed bridge, so just find a pumice formation on the land, cut it to large blocks, tie together and perhaps put some kind of ‘pavement’ (wood or thin tiles of more solid rock) on top and you have a viable bridge (for a couple of years until the pumice soaks and sinks). $\endgroup$
    – Jan Hudec
    Commented Jun 3, 2020 at 6:40
  • $\begingroup$ @JanHudec Find it, cut it, seal it with concrete or tar, then put it in the water? $\endgroup$ Commented Jun 3, 2020 at 12:14
  • $\begingroup$ @JanHudec, pumice is usually found in small pieces -- golf-ball sized or so. It's not so much a matter of "cutting into large blocks" as "gluing into large blocks". $\endgroup$
    – Mark
    Commented Jun 3, 2020 at 22:53
  • $\begingroup$ In the example above the blocks were up to two feet above the surface, implying reasonably large chunks.. Of course, if you have technology or magic to make your own pumice it's a lot easier $\endgroup$ Commented Jun 4, 2020 at 10:31

Expanded clay aggregate already exists and used in construction. It looks pretty much like pebbles, and its density can be as low as 1/4 of that of the water.


As long as it displaces more water than it's own weight it'll float. Concrete barges and rafts exist already.


(This is for educational purposes only. I am not responsible for any damage or injury resulting from what I have described herein. If you do it anyways, please be extremely careful about yourself, other people, and expensive property being exposed to anything I have described herein)

Any object can levitate if it is given enough negative charges of the correct types. Softer negative charges can become "locked-in" the rocks as they are being processed, so they remain floating even when removed from the energy field.

With this type of construction, the engineer is faced with the problem of components floating away into the sky instead of being drawn downward. This may be overcome simply with a large net or wire screen, requiring occasional maintenance to patch the net as well as replacing smaller rocks that have dislodged and floated away upwardly. It may appear ugly, as well as having the wear proportional to usage, similar to the paved roads in that way.

A bridge would be so close to Earth's surface, the vertical gradient of gravity would be negligible. (as opposed to the situation with a floating continent, where the gradient acts as a spring holding it at a certain distance range for a certain load weight)

Over time, the charges will find their way out of the rocks, as they can't be perfectly integrated, similar in principle to a strong magnet losing some of its strength over hundreds of years. The rocks may be "recharged" by putting them through the process again.

Perhaps each rock could be engineered with its own power crystal and altimeter unit, like a scaled-down version of the floating continent system, regulating the charges to maintain position (and load-following the changing distribution of weight, for the army marching across). The smaller crystals needed would be able to react faster, having a much higher operating frequency. They would be capable of developing much more lifting force density than the "natural" type of floating rocks, so you could move many tons of army tanks, troop carriers, missile batteries, etc. across if it ever became necessary

Yet another way could be devised, using special emitters installed near the bridge, constantly beaming the levitation charges into to the rocks. Most would receive equalized charges by conduction, but larger rocks would tend to rotate in irregular patterns if the negative charges become concentrated in the downward-facing side.

The tibetans have caused huge boulders to levitate up the sides of cliffs before, using the same effect differently. Musical instruments were used to resonate the crystalline content of the stone, releasing negative charges within. Occasionally the boulders explode, because too many harder charges with nowhere to go creates more pressure than it can handle (electrostatic repulsion).

By supplying the charges externally, you don't have to use rocks with great enough crystalline content (or the sound power to "activate" them) - any rocks will do, provided they are strong enough for forces encountered in the application.

Please be very careful to make sure the correct types of charges are being put into the floating rocks, for quality of the product and safety of the process. Harder negative charges will be lost to conduction more quickly, or even cause the rocks to explode while being made. Positive charges will have the opposite effect, making the rocks heavier than normal, as well as causing bad sickness for anyone having the positive charges conducted into their body (nuclear radiation).

For more information, see Joseph Cater's book, The Ultimate Reality. It thoroughly explains the physics involved in this type of architecture.

I will demonstrate this principle very soon. Once I have made a video I will share the link here, for everyone to enjoy the demonstration.

  • $\begingroup$ I honestly have no idea whether this is serious or a joke. $\endgroup$
    – Hearth
    Commented Jun 2, 2020 at 21:16
  • 1
    $\begingroup$ Oops, mistook this as physics stack exchange. Sorru $\endgroup$ Commented Jun 2, 2020 at 21:28
  • $\begingroup$ While the answer is a little overboard, the basic principle has been demonstrated, famously in the film "Avatar", where room-temperature superconducting ores created a flux-pinned (quantum-locked) series of floating mountains. The main problem with using this to directly answer the original question as stated is that flux-pinning can only happen over a track or guide of some sort. $\endgroup$
    – Corbin
    Commented Jun 3, 2020 at 0:53
  • 2
    $\begingroup$ @Corbin The answer is also flawed in that the question is asking about structures that float on water, not in the air. And I'm not sure what that part about Tibetans is. A lot of this reads like pseudoscientific nonsense, which is fine in fiction, but the question seems to be asking for realistic methods of doing it. $\endgroup$
    – Hearth
    Commented Jun 3, 2020 at 2:10

Artificial hollow rocks could be designed to float. The problem would be making them "rock like in appearance" and stick together. If you could stand very oddly shaped rocks then the tetrahedral structures used in sea defences might be used if large enough and hollow. They could be reinforced concrete coated with molten rock to finish them off. They would also lock together quite and not float apart.


Buoyancy — the tendency to 'float' - is relatively simple physics. When an object is placed in a liquid like wate) it must displace a mass of liquid; if the gross weight of the object given its volume (its density) is less than the gross weight of water of an equivalent volume, then the object cannot displace enough water to sink below the surface. Thus is floats.

This leaves you with two options for 'floating rocks':

  • (low tech) Lower the density of the rock by hollowing it out and adding air, lighter gases (like helium), or a vacuum. This is how things like ships, pontoons, pumice, and aerogels float; they are constructed so that a large portion of their volume is air, giving them a density lower than that of water. Of course, if one is going to march troops across something like this, the combined density of the stone and everything on it needs to be less than that of water (just as a ship might sink if it is loaded with too much cargo).
  • (ridiculously high tech) Change the gravitational attraction of the rock, so that the force of gravity pulling on it is lessened. If the rock contains materials that are not attracted by (or actively repulse) gravity — some sort of strange material alloy — or has a device that interferes with or controls the planet's gravitational attraction, that would reduce the effective mass so that the rock would be as buoyant as you'd like it to be. The main problem, then, would be to keep the rock dense enough so that it was less buoyant than the surrounding air. You don't want your floating rocks to leave the surface of the water and create a bridge through the sky...
  • 3
    $\begingroup$ Option 2 is not "ridiculously high tech". It's magic. And despite popular sayings they're not the same. $\endgroup$ Commented Jun 2, 2020 at 17:49
  • $\begingroup$ @R..GitHubSTOPHELPINGICE: Eh, I don't disagree; just throwing out possibilities. Sometimes the line between sci-fi and sword & sorcery is productively blurred. Think Star Wars... $\endgroup$ Commented Jun 2, 2020 at 18:17
  • $\begingroup$ @R..GitHubSTOPHELPINGICE, "magic" may also be another name for "sufficiently advanced technology". $\endgroup$
    – Matthew
    Commented Jun 2, 2020 at 19:09
  • 1
    $\begingroup$ @Matthew: See "And despite popular sayings". :-) $\endgroup$ Commented Jun 2, 2020 at 19:13
  • $\begingroup$ @TedWrigley: Yes, I didn't downvote or anything but I don't think option 2 fits well with what the OP was asking for. Despite the lack of tagging, it reads as a hard-science/reality-check type question. $\endgroup$ Commented Jun 2, 2020 at 19:14

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .