Our jaws are dropping.

An increasing number of two-headed sharks have been found in the world’s oceans in recent years, but scientists can’t agree why. And no one is sure how to prevent the trend from inspiring yet another “Sharknado” sequel.

Last month, scientists discovered an egg-laying (oviparous) shark embryo with two heads in a lab in Malaga, Spain. Previously, the only known two-headed sharks were from species that give birth to live babies, according to the BBC.

Anecdotal evidence suggests the number of two-headed sharks has been increasing since 2008, when a fisherman discovered a two-headed blue shark embryo in the Indian Ocean, according to NationalGeographic.com.

In 2011, scientists discovered conjoined twins in blue sharks caught in the Gulf of California and in northwestern Mexico, according to the website. In 2013, fishermen off the coast of Florida caught a female bull shark that was pregnant with a two-headed fetus.

Geobeats via National Geographic This two-headed blue shark was discovered in the Indian Ocean in 2008.

Scientists are having a hard time agreeing why this is happening. Some researchers, like marine scientist Nicolas Ehemann, blame overfishing, according to the Daily Mail.

Ehemann, a master’s student at the National Polytechnic Institute in Mexico, said he thinks the shark gene pool is shrinking, raising the risk of birth defects such as polycephaly, the condition of having more than one head.

Other possible causes include viral infections, metabolic disorders, and pollution, according to NationalGeographic.com.

The increased number of sightings doesn’t necessarily mean there are more two-headed sharks. Popular Science points out that the increased reports may be because more researchers are studying shark embryos and more journals are publishing their findings.