Imagine you are in a mountain cave (in the mountain, or under the earth), and you stumble upon a what seems to be a lake of blood. It's roughly 4mx4m (13ft) wide and 2m (6.5ft) deep. The blood is coming from people's blood being drained directly from them into the pool. Or animal blood, it doesn't really matter to me.

What would this pool look like 500 years later? Would it be crusty, would it have clotted up and rotten away? Would there still be a noticeable stain in the rock? Would there be any way people could find out it was originally a pool of blood?

I'm also okay with adding an anticoagulant (snake venom?). Would that result in the blood evaporating?

Background: I am currently planning a dungeoncrawl for my players, and I want them to find a roughly hemispherical shaped hole in the ground and give them clues about this having been a blood ritual by a forgotten civilisation.

Thank you everyone!

  • $\begingroup$ FWIW you can certainly give context clues (blood channels carved into rocks, sacrificial altar, cave art, etc.) without relying on organic material surviving 500 years. If magic is present then you can might also handwave aspects of decomposition. Remember that 500 is a big number.. e.g. the Boston Massacre was 3.5 centuries ago, we don't expect to find dried blood in the cobblestones. $\endgroup$
    – Cireo
    May 3, 2021 at 18:05

1 Answer 1


Blood, like any other organic substance left to the open in a normal environment, would decompose, no matter if it was coagulated or not.

Based on this paper blood can be detected with various methods up to 15 years after being spilled. None of those methods seems to be viable for a D&D setting, though.

At most you can stretch things a bit out and have the iron contained in the hemoglobin leave a film of iron oxide which, being the common rust, would give a reddish hue to the hole.

My expectation is that normally iron would be consumed by decomposing bacteria and reused, thus none would be left.

On the other hand, if the cave is extremely dry and cold, it can be that the blood dried out and wasn't really decomposed, leaving something observable.

Apparently scientists can recover blood from 500 years old Inca mummies preserved in such conditions. In this case I would expect something dusty.

  • $\begingroup$ Do you know how cold the hole would have to be? Would it be possible to find a cavern that cold, or would it need to be an arctic setting (or similar)? Addition to post: my Campaign is set in a desert landscape, with main parts of it being red because of iron sediments in the rocks. Would the inside of the cavern not be red already? Or would it only be red on the outside bc of its contact with water? $\endgroup$
    – ana_dm
    Nov 17, 2020 at 16:33
  • $\begingroup$ @ana_dm99 close to freezing, and permanently so. Your iron desert is red all over, no water needed. $\endgroup$
    – Trish
    Nov 17, 2020 at 16:40
  • $\begingroup$ @Trish do you by any chance know how cold a system of tunnels/caverns under the surface could get in a desert? $\endgroup$
    – ana_dm
    Nov 17, 2020 at 16:52
  • $\begingroup$ @ana_dm99 depends on the desert! The Gobi is surprisingly wet for a desert, Antarctica is technically a desert with an average temperature below 0°C... $\endgroup$
    – Trish
    Nov 17, 2020 at 16:57
  • $\begingroup$ @Trish hmmm what about the Sahara? What does the overall temperature influence? I'm trying to make stuff make sense, and I don't jsut want to blindly guess at something. $\endgroup$
    – ana_dm
    Nov 17, 2020 at 17:07

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