Underground Sea is the most expensive card from Revised Edition (April 1994) – the first Core set which did not include the Power Nine – standing at no less than 250,00 € each. And in the preceding sets, it is also firmly placed in the top ten most expensive cards, sometimes even outranking individual Power Nine cards. This is due to the original blue / black dual land being one of the very best lands ever printed.
Fundamentally, the upside of Underground Sea is the same as those of the other nine dual lands from Limited Edition Alpha (August 1993), Limited Edition Beta (October 1993), Unlimited Edition (December 1993), and Revised Edition (ABUR). These ten cards each have two basic land types and none of the drawbacks used to defang the later dual lands, such as entering the battlefield tapped.
When the two land types just happen to be an Island and a Swamp, as is the case with Underground Sea, it becomes clear why the card outclasses its competition amongst the ABUR dual lands. This combination produces the mana required for some of the most powerful spells in the Eternal formats Vintage and Legacy, making Underground Sea an obvious 3- to 4-of in almost any deck requiring both blue and black mana. The card is a natural include in U/B Delver, Grixis Control, and Storm. It even sees play in U/B Shadow where it may be fetched rather than a Watery Grave if the player’s life total is at critical level.
The major factor driving up the price of Underground Sea is unsurprisingly the fact that the card is on the Reserved List. With no reprints possible, new copies of the card are prevented from entering the secondary market keeping the supply at best at a constant while demand only seems to be increasing. Even if the card is only legal in Eternal formats such as Vintage, Legacy, and Commander as well as fringe formats including Old School and Canadian Highlander, there are still always buyers ready to pay a premium for access to the speed and flexibility that the original dual land provide.
Underground Sea was played in more than 30.0% of Vintage decks participating in live tournaments in the first half of 2019 (according to MTG Top 8), making it the most played of the original ABUR dual lands in the format for this period. It remains less played than the blue fetches Scalding Tarn, Misty Rainforest, Polluted Delta, and Flooded Strand with the last two played in more than 40.0% of decks. But to a large extend, the popularity of these four lands is for a large part, due to their ability to fetch exactly the Underground Sea.
Underground Sea seems to be experiencing a dip in popularity in the current meta (medio 2019), at least in Legacy, where the trend appears to be headed downwards – from the card being included in 32.0% of all decks ever registered in MTG Top 8 to 24.3% for the first half of 2019 and a meager 21.0% for May and June 2019. This may contribute to the lower and somewhat more stable price experienced by the card after a massive spike around mid-2018.
However, predicting that card will once again see the price levels experienced during the 2018 craze (and possibly even higher) seems to be a no-brainer. Excluding unforeseeable and currently unthinkable events, such as the ending of the Reserved List or Wizards of the Coast veering from well-established design axioms of not printing dual lands with no downsides compared to basic lands, Underground Sea will remain irreplaceable and in high demand.