The Indus Valley civilization was one of the most advanced in the world for more than 500 years, with more than a thousand settlements sprawling across 250,000 square miles of what is now Pakistan and northwest India from 2600 BCE to 1900 BCE. It had several large, well-planned cities like Mohenjo-daro, common iconography—and a script no one has been able to understand.

Over at Nature, Andrew Robinson looks at the reasons why the Indus Valley script has been so difficult to crack, and details some recent attempts to decipher it. Since we don't know anything about the underlying language and there's no multilingual Rosetta stone, scholars have analyzed its structure for clues and compared it to other scripts. Most Indologists think it's "logo-syllabic" script like Sumerian cuneiform or Mayan glyphs. But they disagree about whether it was a spoken language or a full writing system; some believe it represented only part of an Indus language, Robinson writes.

One team has created the first publicly available, electronic corpus of Indus texts. Another, led by computer scientist Rajesh Rao, analyzed the randomness in the script's sequences. Their results indicated it's most similar to Sumerian cuneiform, which suggests it may represent a language. Read the full article for more details.

The Indus Valley script is far from the only one to remain mysterious. Here are eight others you might try your hand at deciphering.

1. Linear A

In 1893, British archaeologist Sir Arthur Evans purchased some ancient stones with mysterious inscriptions on them at a flea market in Athens. On a later trip to the excavations at Knossos on the island of Crete, he recognized one of the symbols from his stones and began a study of the engraved tablets being uncovered at various sites on the island. He discovered two different systems, which he called Linear A and Linear B. While Linear B was deciphered in the early 1950s (it turned out to represent an early form of Greek), Linear A, above, has still not been deciphered.

2. Cretan Hieroglyphics





The excavations on Crete also revealed a third type of writing system, with symbols that looked more picture-like than those of the linear scripts. Some of these symbols are similar to elements in Linear A. It is assumed that the hieroglyphic script developed into Linear A, though the two systems were both in use during the same time period.

3. Wadi el-Hol script





In the 1990s, a pair of Yale archaeologists discovered a graffiti-covered cliff wall at the Wadi el-Hol (Gulch of Terror) in Egypt. Most of the inscriptions were in systems they could recognize, but one of them was unfamiliar. It looks like an early transition from a hieroglyphic to an alphabetic system, but it hasn't yet been deciphered.

4. Sitovo inscription





In 1928 a group of woodcutters found some markings carved into a Bulgarian cliffside. They thought the marks indicated hidden treasure, but none was found. Word got around and soon some archaeologists had a look. Later, the head of the expedition was executed for being a secret agent for the Soviets in Bulgaria. One piece of evidence used against him was a strange coded message he had sent to Kiev—actually a copy of the cliffside inscription he had sent to colleagues for scholarly input. It is not clear what language the inscription represents. Thracian, Celtic, Sarmato-Alanian, and Slavic are some of the possibilities scholars have argued for. Another suggestion is that it's simply a natural rock formation.

5. Olmec writing

The Olmecs were an ancient Mexican civilization best known for the statues they left behind: the so-called "colossal heads." In 1999, their writing system was revealed when road builders unearthed an inscribed stone tablet. The tablet shows 62 symbols; some look like corn or bugs, and some are more abstract. It has been dated to 900 B.C., which would make it the oldest example of writing in the Western Hemisphere.

6. Singapore stone





There once was a giant engraved slab made of sandstone at the mouth of the Singapore River. It had been there for 700 years or so when, in 1819, workers uncovered it while clearing away jungle trees. A few scholars got a look at it before it was blown to bits in order to make space for a fort to protect the British settlements. The parts that didn’t end up in the river were eventually used for road gravel, though some fragments were saved. The script hasn't been deciphered, but there have been various suggestions for what language it might represent: ancient Ceylonese, Tamil, Kawi, Old Javanese, and Sanskrit.

7. Rongorongo





When missionaries got to Easter Island in the 1860s, they found wooden tablets carved with symbols. They asked the Rapanui natives what the inscriptions meant, and were told that nobody knew anymore, since the Peruvians had killed off all the wise men. The Rapanui used the tablets as firewood or fishing reels, and by the end of the century they were nearly all gone. Rongorongo is written in alternating directions; you read a line from left to right, then turn the tablet 180 degrees and read the next line.

8. Proto-Elamite

This ancient writing system was used more than 5000 years ago in what is now Iran. Written from right to left, the script is unlike any other ancient scripts; while the proto-Elamites appear to have borrowed the idea for a written language from their Mesopotamian contemporaries, they apparently invented their own symbols—and didn't bother to keep track of them in an organized way, proto-Elamite expert and Oxford University scholar Jacob Dahl told the BBC in 2012. Around that time, he and his Oxford colleagues asked for help from the public in deciphering proto-Elamite. They released high-quality images of clay tablets covered in Proto-Elamite, hoping that crowdsourcing could decode them. Now a collaboration involving several institutions, the project is ongoing.