1st gen yugioh cards Orica Introduces the Mainspring and Bugman archetypes. Beckett: Online Sports & Non

This combination of demand and rarity makes Mana Crypt the 4th-most-valuable card outside the Reserved List, behind two cards from Portal: Three Kingdoms (Imperial Seal and Zodiac Dragon) and one (Grim Tutor) which was exclusive to a particular 1999 starter deck

You can untap it by putting a -1/-1 counter on it, so pair it with something that offsets the counters or turns it into a non-creature, and youve got infinite green mana

Not everyone is going to have six creatures on the battlefield, so this is a battlefield sweeper in many cases

If you have Unown in the Active spot, you just need to bench your Klefki one last time after your last loop in order to reach the required 66 counters