Your manabase should be built alongside the rest of your deck, not thrown together or copied from a stock decklist without thought. Most D&T manabases begin with the core of ~10 Plains, 4 Wasteland, 4 Rishadan Port, and 2-3 Karakas. Sometimes players play a split of Snow-Covered Plains and Plains to play around the card Predict. After that, pilots play a few flex lands based on personal preference. The vast majority of D&T decks are going to want 23 lands, though there are some cases where going up to 24 is appropriate (e.g. playing multiple four drops). Going down to 22 lands is a risky proposition, but occasionally a 22 land list posts results.
I’ve listed some of the most common options for D&T manabases below. Of note, every non-Plains land you add to your deck further exposes you to cards like Price of Progress, Blood Moon, Wasteland, and Back to Basics; however, those lands add utility that often is worth that cost, and as an added bonus, help you to play around Massacre.
Horizon Canopy
Horizon Canopy hedges well against flooding out in the late game. The amount of damage it deals to you is usually negligible. If you compare it to the damage that most Legacy decks end up taking from their fetch lands, it doesn’t seem unreasonable or out of the ordinary. When you are land-light or need to be Porting regularly, Horizon Canopy can be a bit awkward. Of note, Horizon Canopy is especially great when played alongside Enlightened Tutor, as it is a way to draw the card immediately.
Horizon Canopy tends to be great in grindy matchups. It serves as an early game mana source and then you can cash it in for a redraw later in the game. It is at its worst in the Burn matchup. While the life loss is usually negligible, you often stabilize the Burn matchup at 6 or less life. A few Horizon Canopy activations can literally be the difference between winning and losing that matchup.
Cavern of Souls
While making your creatures uncountable is a nice perk, it’s not particularly important for most matchups. These days Cavern of Souls is used in flex slots to force critical cards to resolve in certain matchups or as a mana fixer for splash builds.
You are typically going to name human with Cavern of Souls to get the most value, but that is not always the correct play. For example, sometimes you’ll need to name Elemental to get a Flickerwisp in play, which can then reset your Cavern.
Cavern is not a true white source. It does not cast cards like Swords to Plowshares, Council’s Judgment, and Gideon, Ally of Zendikar. It makes hands with multiple creatures types and double white mana costs awkward. Playing Cavern is a calculated risk: you make your blue matchup slightly easier, but sometimes will not have the colored mana you need.
Mishra’s Factory
Mishra’s Factory is an additional threat that is great at putting additional pressure on decks without many opposing creatures. Against combo decks, it can often decrease the clock by a turn or two. Against control, it is a solid plan B for when your board gets wiped with a sweeper. In the late game, it can pick up equipment and close out the last few points of damage pretty easily. It is pretty mediocre in creature mirrors, as something as simple as a Shardless Agent keeps it from attacking. Since it can block and then tap, turning into a 3/3, it can block 2/2 creatures and survive. Playing one Mishra’s Factory is usually fine, but playing a second one starts to really decrease your ability to cast colored spells.
Flagstones of Trokair and Dust Bowl
Flagstones of Trokair is not worth playing unless you have some way to abuse it. It does not do enough as a deck thinning card or as a way to hedge against Massacre. Flagstones is best in decks that play multiple Cataclysms or Armageddons in the sideboard. Both become much stronger sideboard cards when you have the ability to rebuild more quickly after casting them.
Alternatively, Flagstones can be a devastating late-game Wastelanding machine when paired with Dust Bowl. If you want to play the Dust Bowl and Flagstones engine, it may be worth going up to a 24th land. This engine is extremely slow, but it can grind out some midrange decks rather effectively when the board is otherwise at parity.
Sea Gate Wreckage
Sea Gate Wreckage is an interesting possible inclusion in the deck. It provides a late game card advantage engine that is very useful for matchups like Shardless BUG and Jund. In most matchups, however, it will just be a non-basic, colorless source for all or most of the game. My initial testing with this card has shown it to be a touch too slow, but this is still in my “I’m not going to forget this exists” pile. Of note, if you control two of these, you can activate one and respond to the activation by activating another to draw two cards.
Ghost Quarter
Ghost Quarter is an acceptable budget substitution for Rishadan Port on MTGO. Against some decks like Eldrazi, Lands, or Delver, it can just functionally be Wastelands 5-8. It is going to be far worse than Rishdan Port against decks that have an abundance of basics, but it is certainly playable. Ghost Quarter is also sometimes played alongside Leonin Arbiter or Aven Mindcensor for the lockout potential.