The complexity of basic lands

One could find a bit of irony in the fact that basic lands are both the most ubiquitous and simplest types in Magic the Gathering, yet at the same time there's a great deal of hidden complexity behind them. (The complexity does not come from basic land cards themselves, but from the concept of "basic land type," which has a good amount of implicit rules behind it.)

Assume that Urborg, Tomb of Yawgmoth and Blood Moon are on the battlefield at the same time. What happens?

If you remember from my previous posts, or as you may already know, if an effect sets, for example, the creature subtype of a creature permanent to something, it will only replace the existing creature subtypes of said permanent and nothing else. (It will not affect any other types or supertypes the permanent might have, eg. if it's an Artifact or it's Legendary, nor will it affect the subtypes of other types, such as it having a land subtype. More importantly, though, it will not have any effect on any existing abilities or other characteristics of the permanent.)

These two cards do not set the type of lands to anything. They only set their land subtype (to Swamp and Mountain respectively.) However, these are basic land subtypes, and this is governed by special rules: If an effect sets the subtype of a land to one of the basic land subtypes (ie. Forest, Plains, Mountain, Swamp or Island), that land will lose all of its existing land subtypes and all of its existing abilities (unless the effect says "in addition to its other types") and gains the ability of tapping for one mana of the respective color. The loss of existing abilities has quite radical implications when anything sets the land subtype to that of a basic land.

To take a simpler example, consider a Sulfurous Springs enchanted with Lush Growth.

What happens here? Lush Growth is setting the land subtypes of Sulfurous Springs to Mountain, Forest and Plains. This means that it loses its existing abilities (which means you cannot tap it for colorless mana nor for black or red mana and taking 1 damage) and gains three different abilities: The first ability to tap for one red mana, the second to tap for one green mana and the third to tap for one white mana.

(Note that while Sulfurous Springs now has basic land subtypes, it's not itself a basic land. "Basic" is a supertype, and Lush Growth does not grant it.)

Before we go to the question at the beginning, consider the situation where these latter two cards are on the battlefield, and in addition there is also a Blood Moon (which says that "nonbasic lands are Mountains.") What happens?

This is a situation where we have two type-setting effects on the same layer (layer 4) affecting the same permanent. Therefore it's a question of timestamps. In other words, the answer depends on which happened first, Blood Moon entering the battlefield or Lush Growth enchanting Sulfurous Springs. The one that has the newer timestamp trumps the other.

So now to the original question: What happens when both Urborg, Tomb of Yawgmoth and Blood Moon are at the same time on the battlefield? Are nonbasic lands Swamps (in addition to their existing types) or Mountains (with no other types and abilities)? Are they both? Are basic lands Swamps in addition to their own types?

It may sound at first like this is also a question of timestamps, ie. that it depends on which one entered the battlefield first. However, there's a crucial difference here: Urborg is itself a land, so Blood Moon is also affecting it. (If Blood Moon sets Urborg's subtype to Mountain, it loses all of its abilities, which means it will not make anything a Swamp.)

This is a rarer situation where there are dependent effects on the same layer. This is a special exception to the timestamp rule which states that if the text or existence of one effect on a layer would depend on another effect on the same layer, the affecting effect is evaluated before the affected one (regardless of timestamps.)

In this particular case Blood Moon's effect is evaluated first regardless of timestamps because it modifies Urborg's effect. Therefore Urborg becomes a Mountain and loses its abilities and doesn't make anything a Swamp. (In other words, all nonbasic lands will be Mountains and all basic lands will be unmodified.)