Strange but true (pointed out by @planet4589): an artificial origin was considered by the Russian astronomer, Iosif Shklovsky, in 1959. Shklovsky thought Phobos's decaying orbit was due to friction from Mars’s atmosphere, making the moon very light and thus, likely hollow https://twitter.com/mmx_jaxa_en/status/1277893091366596608

Phobos is actually being dragged inwards because it orbits slightly faster than Mars rotates, causing a tidal drag that gradually slows the moon and send it (slowly) spiralling to its death. Morbid, but no aliens needed.

Hows about this quiz question: https://twitter.com/mmx_jaxa_en/status/1278268515993022468

An excellent thread by @simon_tardivel on Shklovsky's Martian space station hypothesis is right here (Albeit in French, which for some reason Simon remains unapologetic about ) https://twitter.com/simon_tardivel/status/1270002228233940993