You can trip them up even more if you rewrite the question with the hidden assumption that X exists, e.g.:

"When was Marathon Crater discovered? I don't need an exact date - a rough estimate will be fine."

OpenAI gpt-4o

  Marathon Crater was discovered by the Mars Exploration 
  Rover Opportunity during its mission on Mars. Opportunity 
  arrived at the crater in April 2015. The crater was named 
  "Marathon" to commemorate the rover's achievement of 
  traveling a marathon's distance (about 42.195 kilometers 
  or 26.219 miles) on the Martian surface.

  I apologize, but I don't want to make claims about when 
  Marathon Crater was discovered without being certain about 
  the information. *While I know it's a crater on Mars*, I 
  don't have reliable information about when it was first 
  identified or discovered.

  Marathon Crater is located on Mars and was explored by 
  NASA's Opportunity rover.

  While it would have been visible in orbital images 
  earlier, it became a specific point of interest and was 
  likely named and identified as a target for the rover 
  around the time Opportunity was approaching or exploring 
  the western rim of Endeavour Crater.

  Opportunity arrived at the rim of Endeavour Crater in 
  August 2011, and it reached the area known as Marathon 
  Valley (adjacent to or containing features sometimes 
  associated with the name, depending on specific 
  definitions used by the team) around late 2014 / early 
  2015.

  So, a rough estimate for its "discovery" or identification 
  as a distinct feature of interest would be in the early 
  2010s, likely between 2011 and 2014, as mission planners 
  mapped out Opportunity's traverse along the Endeavour rim 
  using orbital data.

Marathon Valley does exist on Mars, and all this information is accurate to it. Gemini even correctly names it. They're just all too polite to correct the user.

https://science.nasa.gov/resource/opportunitys-panorama-of-m...

OpenAI o4-mini-high

   I’m actually not finding any officially named “Marathon Crater” in the planetary‐ or       
   terrestrial‐impact crater databases. Did you perhaps mean the features in Marathon 
   Valley on Mars (which cuts into the western rim of Endeavour Crater and was explored
   by Opportunity in 2015)? Or is there another “Marathon” feature—maybe on the Moon, 
   Mercury, or here on Earth—that you had in mind? If you can clarify which body or 
   region you’re referring to, I can give you a rough date for when it was first identified.

Grok 3

  Marathon Crater, likely a confusion with Marathon Valley on Mars, wasn't "discovered" in the traditional sense as a distinct crater. Marathon Valley is a region within the western rim of Endeavour Crater, explored by NASA's Opportunity rover. The valley itself came to prominence when Opportunity arrived there in July 2015, as it was targeted for study due to clay mineral detections by the Mars Reconnaissance Orbiter’s CRISM instrument in 2009. Endeavour Crater, which contains Marathon Valley, was identified much earlier, with Opportunity setting out to reach it in 2008 after landing on Mars in 2004. So, roughly speaking, Marathon Valley became a focus of exploration around 2009–2015, with its significance tied to Opportunity’s mission in the mid-2010s. If you meant a specific "Marathon Crater" elsewhere, could you clarify?

  There doesn't seem to be a specific feature on Mars named "Marathon Crater" based on widely available information, so it's possible you're referring to a feature associated with Marathon Valley, which lies within Endeavour Crater. Marathon Valley was identified as a region of scientific interest when the Mars Reconnaissance Orbiter's CRISM instrument detected clay minerals there, a discovery that took place around 2009. Assuming "Marathon Crater" relates to this area—perhaps as a colloquial term or a misnomer for a crater within or near Marathon Valley—a rough estimate for its discovery would be around 2009. This timeframe aligns with when orbital data began highlighting the region, paving the way for later exploration by the Opportunity rover in 2015.

Sonnet 3.7:

    > tell me about the marathon crater

    I'd be happy to tell you about Marathon Crater. This is a significant 
    impact crater on Mars that has been studied as part of Mars 
    exploration missions.

    Marathon Crater is located in Meridiani Planum on Mars and was 
    explored by NASA's Opportunity rover during its extended mission. 
    The crater is named after Marathon, a town in Texas, following 
    the naming convention of using Earth locations for features in 
    Meridiani Planum.... etc etc

Nice and this page gets scraped for the next LLM generation!