The Mysterious Bones (怪骨 of various creatures appear on Kaikotsu)Infant Island in the 1964 Godzilla film Mothra vs. Godzilla. A skeletal creature among the other remains, resembling a turtle, seems to still be moving.
The graveyard of bones is known only by the name Kaikotsu (怪骨), translating to "Mysterious Bones." This name seemingly originated with a pair of officially licensed ornaments released by Cast in 2012; the skeletal turtle-like creature was fully named Mysterious Bones of Infant Island (インファント島の怪骨, while a group of other skeletons was called Other Mysterious Bones of Infant Island Infanto-tō no Kaikotsu) (インファント島の別の怪骨. Officially licensed T-shirts were also sold of the former, translating the "Mysterious Bones of Infant Island" name into English as Infant Island's Kaikotsu. This skeletal turtle, which has become particularly popular among fans, was nicknamed "Skeleturtle" by Chris Mirjahangir of Infanto-tō no Betsu no Kaikotsu)Toho Kingdom before the official name was known. Another unofficial fan nickname for the creature, "Kame no Gaikotsu" (亀の骸骨, lit. "Turtle Skeleton"), was coined by Miles Imhoff, also from Toho Kingdom.
Though it is unknown exactly who decided on or created the skeletons seen in the final film, the turtle-like Mysterious Bones may have been included as a reference to the 1962 Italian documentary Mondo Cane (A Dog's World), originally released in the United States as A Dog's Life. The film contains a large number of documented atrocities to nature and animals, including turtles on the radiation-contaminated Bikini Atoll beaches, many of which were dead or dying from the continued fallout from nuclear tests. The dried bones of the turtles move in the wind, similar to the Mysterious Bones' movement.
The turtle-like creature among the Mysterious Bones is purely white and gray, with black eyes and a bland, pale shell with no visible scutes. It has a fairly long neck, and very short limbs, which have tiny bone fragments visible. Though it could be interpreted as either a skeleton bobbing in the wind or a living creature moving its head back in forth in Mothra vs. Godzilla, it is unambiguously portrayed as a living creature in its brief appearance in Godzilla: Rulers of Earth. Other Mysterious Bones which appear include a giant skull and rib cage connected by a miniature spine, and several smaller birdlike skulls, some of which possess visible appendages.
Within the context of Mothra vs. Godzilla, the exact origins of the Mysterious Bones are unknown, though they are presumably the result of radioactive fallout on Infant Island due to repeated nuclear bomb tests affecting the local wildlife. It is unclear whether the turtle-like formation of Mysterious Bones is a living creature or simply the skeletal remains of one like the other bones around it.
When Ichiro Sakai, Junko Nakanishi, and Professor Shunsuke Miura traveled to Infant Island to beg Mothra for help in defeating Godzilla, they witnessed how the island had been devastated by nuclear tests. The island was littered with the skeletal remains of numerous creatures, some giant, which had been killed in the fallout. One particular skeleton, resembling a large turtle, sat among the remains scattered around the group's landing site and swayed its head back and forth as if alive. As the group traveled up a ridge, they found yet more skeletons before being ambushed by a party of Infant Islanders.
- Godzilla: Rulers of Earth #19 (2014)
- Godzilla: Monsters & Protectors - All Hail the King! #5 (2023)
When Cedric Nishimura updated the viewers of his MeToob channel on Godzilla and King Caesar's victory over King Ghidorah, a thumbnail to the right of the video, partially cut off on the webpage, showed the word "Mystery" and a picture of a creature which resembled the turtle-like Mysterious Bones.
- Main article: Mysterious Bones/Gallery.
- Despite its obscurity, the Mysterious Bones received two officially licensed ornaments from Cast in 2012. The turtle-like skeleton also received a T-shirt and later became one of Cast's first three miniature ornaments in 2018.
- The turtle-like creature among the Mysterious Bones appears in the officially-licensed Godzilla book Shinji Nishikawa: Drawing Book of Godzilla (2016), as sketches made by Shinji Nishikawa on the inner back cover and last page of the book.
- A T-shirt given to crew members of Godzilla x Kong: The New Empire (2024), illustrated by Christian Gonzalez, features the turtle-like skeleton.
This is a list of references for Mysterious Bones. These citations are used to identify the reliable sources on which this article is based. These references appear inside articles in the form of superscript numbers, which look like this: 
- Toho SF Special Effects Movie Series Vol. 2: Mothra/Mothra vs. Godzilla. Toho. 1 January 1985. ISBN 4-924609-04-8.