Strange, unexplained flashes have been seen amid a massive earthquake in Mexico.

The flashes – which resemble lightning – lit up the sky after the tremor hit Mexico City and elsewhere. The earthquake was the strongest to hit the country in 100 years, triggered a tsunami warning, and has already led to more than 30 deaths.

And the intense shakes also led to strange blue and green flashes lighting up the night sky as people fled into the streets to avoid the danger.

Nobody is entirely clear why the bright lights actually happen, though they have been reported for years. Some argue that they are connected to the energy being released from the ground, while others say they are the result of secondary effects like explosions from power stations or electricity cables.

The mystery of the flashes has even led people to think of them as supernatural. "In the past, people often interpreted [earthquake lights] in religious terms, and in modern times they thought of UFOs, although there is a completely rational physical explanation that we are working on," Friedemann Freud, who co-authored a study about the lights, told National Geographic.

Professor Freud's study claimed that the lights came about because the electrical properties of rocks on the ground. The energy from the tremors release electrical charges from the rocks, which can then show bright blue and green lights across the sky in the wake of a tremor.