ACT English Passage 03: Blue Holes of the Bahamas

[1] The Bahamas, a series of semitropical islands off the southeast coast of the United States, which are (31) home to some of the most unusual geological formations in the world: underwater caves known as blue holes. [A] These vertical caves were formed over thousands of years, and their cold depths provide abundant clues to the islands’ past.

[2] (32) During the formation process, tiny grains of calcium carbonate separated from the seawater. These grains built (33) up, then compacted, forming the limestone that makes up the islands. [B] Over time, rainwater permeated the porous limestone but was trapped just above sea level, buoyed by (34) the denser seawater below. Jostled back and forth by tides, the layer of slightly acidic, brackish water eroded limestone faster than either rainwater — or seawater — (35) could alone. As the limestone eroded caves (36) formed.

[3] Over time periods in which the weather changed drastically, (37) sea levels rose and fell by hundreds of feet. This allowed the cave-creating process to be a process that repeated at different depths hundreds of feet apart (38). The roofs of many caves collapsed, leaving the chambers beneath exposed.

Some of these blue holes open to small contained caves (39) others open to miles-long interconnected tunnels. (40)

[4] The telltale sign of a blue hole is a circular patch of water striking darker (41) than the water surrounding them (42). (The darker water indicates greater depth.) [C] Divers have found the remains of turtles and alligators. Now extinct on (43) the islands, stalactites and stalagmites from a time when the caves were above sea level, and artifacts of early human inhabitants.

[5] [D] Hundreds of blue holes can be sighted off (44) the Bahamas. So far, most remain unexplored by divers, owing in part to the danger of cave diving. Often the only clue to the mysteries below is the tantalizing sight of dark blue water leading deep into the sea.


B. States, are
C. States are
D. States,

32. Given that all the following statements are true, which one, if added here, would most effectively introduce the topic of this paragraph?
F. The Bahamas were formed from calcium carbonate, a component of seawater.
G. Calcium carbonate, a common rock substance, is also found in seawater.
H. Much of the land making up the Bahamas is still underwater.
J. Most types of limestone contain calcium carbonate.

B. are building
C. will build
D. build

G. being buoyed because of
H. it being buoyed by
J. buoying it was

B. rainwater, or seawater,
C. rainwater, or seawater
D. rainwater or seawater

G. limestone, eroded caves
H. limestone eroded, caves
J. limestone eroded caves,

37. Which choice most specifically illustrates how long the cave-creating process took?
B. Between ice ages and the more temperate eras that followed them,
C. During this extended time,
D. As time passed,

G. repeat again and again at various different depths.
H. repeat at different depths that varied.
J. repeat at different depths.

B. caves that
C. caves;
D. caves,

40. At this point, the writer is considering adding the following true sentence:

At 663 feet deep, Dean’s Blue Hole in Long Island, Bahamas, is a popular cave-diving destination.

Should the writer make this addition here?
F. Yes, because it supports the preceding sentence by providing an example of a blue hole that is very deep.
G. Yes, because it allows the reader to visualize a specific blue hole in the Bahamas.
H. No, because it offers a detail that is unrelated to the paragraph’s focus on the cave-creating process.
J. No, because it does not provide an adequate description of Dean’s Blue Hole.

B. strikingly darker
C. strikingly darkly
D. striking darkly

G. these.
H. one.
J. it.

B. alligators now. Extinct on
C. alligators now extinct on
D. alligators now extinct. On

44. Which choice most effectively suggests the shape of blue holes as described earlier in the essay?
G. dot the waters of
H. darken parts of
J. appear in

Question 45 asks about the preceding passage as a whole.

45. The writer wants to add the following sentence to the essay:

In these depths, fossils and ancient rock formations are incredibly well preserved.

This sentence would most logically be placed at:
A. Point A in Paragraph 1.
B. Point B in Paragraph 2.
C. Point C in Paragraph 4.
D. Point D in Paragraph 5.


31. The best answer is B because “a series of semitropical islands off the southeast coast of the United States” is an appositive phrase that further describes the Bahamas. This phrase is nonrestrictive and must be set off by commas. The verb are is also needed to avoid a sentence fragment.

32. The best answer is F because this paragraph focuses on the formation of the caves, and this choice most logically introduces the topic by introducing a critical component
of the formation: calcium carbonate. This choice is the only one to give us two pieces of essential information: that the Bahamas were formed from calcium carbonate and that calcium carbonate is a component of seawater.

33. The best answer is A because the simple past tense built is consistent with the verb tense in the rest of the paragraph.

34. The best answer is F because this choice presents the clearest, most concise, and most idiomatic way to say that the rainwater was buoyed by the seawater below.

35. The best answer is D because the word either in the sentence indicates that the brackish water is being compared to two things at the same time: rainwater and seawater. The omission of punctuation is appropriate and indicates that these two nouns are parallel elements in the construction “either rainwater or seawater.”

36. The best answer is H because the introductory adverb clause “As the limestone eroded” must be separated from the main clause (“caves formed”) by a comma.

37. The best answer is B because this choice provides the most specific indication of time. Both “ice ages” and “temperate eras” speak to large swaths of time and indicate that a great deal of time passed during the cave-formation process.

38. The best answer is J because the phrase “repeat at different depths” is the clearest and most concise choice in this context.

39. The best answer is C because the semicolon correctly separates the two independent clauses.

40. The best answer is H because the specific information about Dean’s Blue Hole as a popular cave-diving destination is not related to the focus of the paragraph, which is on cave formation in general.

41. The best answer is B because the noun water must be modified by the adjective darker, and the adjective darker should be modified by the adverb strikingly. This is
the only choice that uses the correct adjective and adverb forms of these words.

42. The best answer is J because the pronoun it is appropriate given the context and agrees in number with its antecedent “a circular patch of water.

43. The best answer is C because the sentence presents a list of three findings of divers, the first of which is “remains of turtles and alligators now extinct on the island.”

44. The best answer is G because the essay tells us that the telltale sign of a blue hole is a “circular patch of water.” The word dot (a small round mark) indicates the circular
shape of the blue holes and is the only choice that suggests a shape.

45. The best answer is C because this placement most logically connects “these depths” in the added sentence to the first two sentences of paragraph 4 in which the depths of the water are described. The information about the well-preserved fossils and rock formations also helps introduce other discoveries made by divers in the caves.