1. What was discovered on Mars?

3. Fish-shaped rock

Source: NASA/JPL/Circle

We do not have enough evidence yet, which suggests that there is life on Mars. But UFO and alien enthusiasts keep finding signs of life in optical illusions and pareidolia. 

NASA's Curiosity rover photographed one such unusual formation that looked like a fish. While we cannot confirm whether it was a fossil or simple rock, it is safe to assume the latter.
NASA has previously stated that "Mars likely never had enough oxygen in its atmosphere and elsewhere to support more complex organisms. Thus, large fossils are not likely."

4. Jelly Donut

Source: NASA/JPL-Caltech/Cornell Univ./Arizona State Univ.

The Opportunity rover captured two subsequent images showing the sudden appearance of what looked like a jelly-doughnut. People started identifying it as an alien fungus, and the theory quickly spread in conspiracy theory circles.

The real explanation came a little later when NASA announced that the rock's sudden appearance was due to the rover dislodging it while driving.

5. Blueberries

Source: NASA/JPL-Caltech/Cornell/USGS

Another discovery by the Opportunity rover was the ‘blueberries.’ These BB-sized pebbles are hematite-rich spherules. 

They were first discovered in 2004 near the Fram Crater, but there is no proven hypothesis about how they got the shape.

6. Spheres

Source: NASA/JPL-Caltech/Cornell Univ./ USGS/Modesto Junior College

Opportunity discovered another unusual formation in 2012 at the Kirkwood outcrop. These spherical rocks are different from the blueberries found earlier. 

They are a puzzling discovery, according to NASA, with respect to their concentration, distribution, and structure.

7. Dust Devil

Source: NASA/JPL-Caltech

Opportunity ran across a peculiar jet that appears in the image of the Mars landscape photo in 2016. It's a dust devil, not unlike the ones found on Earth. 

These dust devils are potential weather hazards for human visitors, and the Mars missions of the future will need a solution to deal with them.

8. Avalanche

Source: NASA/JPL/University of Arizona

NASA's MRO found this phenomenon in 2010. Here, a cloud of particles is puffing upward at a steep cliff.

The probable cause is an avalanche of CO2 frost. These are a common occurrence in the springtime of the planet.