The planet is undergoing a sixth mass extinction, the sixth time in the history of life on Earth that global fauna has experienced a major collapse in numbers.

Historically, mass extinctions have been caused by catastrophic events like asteroid collisions. This time, human activities are to blame.

The primary culprits are deforestation, mining, and carbon-dioxide emissions, which cause the planet to heat up.

As a result, insects are dying off at record rates, animal species are experiencing "biological annihilation," and invasive aliens are driving native species to extinction.

The phrase "mass extinction" typically conjures images of the asteroid crash that led to the twilight of the dinosaurs.

Upon impact, that 6-mile-wide space rock caused a tsunami in the Atlantic Ocean, along with earthquakes and landslides up and down what is now the Americas. A heat pulse baked the Earth, and the Tyrannosaurus rex and its compatriots died out, along with 75% of the planet's species.

Although it may not be obvious, another devastating mass extinction event is taking place today — the sixth of its kind in Earth's history. The trend is hitting global fauna on multiple fronts, as hotter oceans, deforestation, and climate change drive animal populations to extinction in unprecedented numbers.

Read more: Insects are dying off at record rates — an ominous sign we're in the middle of a 6th mass extinction

A 2017 study found that animal species around the world are experiencing a "biological annihilation" and that our current "mass extinction episode has proceeded further than most assume."

Here are 12 signs that the planet is in the midst of the sixth mass extinction, and why human activity is primarily to blame.