1. What was discovered on Mars?

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.
9. Waffle-shaped island
Source: NASA/JPL-Caltech/University of Arizona
Another image from MRO taken in 2014 shows an island in the shape of waffles. It is 1.2-miles wide and is located in an area of lava flows.
This lava could be causing the unusual sight but pushing the formation upwards.