Normally the earth is a Planet floating in space, so this was a special case…:
“Bhārata-varṣa: This part of the world is also one of the nine varṣas of the Jambūdvīpa, or earthly planet. Each planet is also sometimes called a dvīpa because of its being an island in the fathomless outer space. Each planet is factually an island in the airy ocean of outer space. Jambūdvīpa is only one of such countless islands in this airy ocean of space. A description of Bhārata-varṣa is given in the Mahābhārata (Bhīṣma Parva, Chapters 9-10).”
[SB 1.16.12 Purport REAL SB Scan and not the slightly changed one from the Vedabase2003″]

Mukunda Das has destroyed the flat earth scam: http://truth.prabhupada.org.uk/category/the-flat-earth-agenda
Here Prabhupada says it is inconceivable:
Prabhupāda: Ask them. Why don't you ask them? Sometimes they say flat, sometimes they say it is round. Why don't you ask them, the scientists?

Yaśodā-nandana: We don't accept what they say.

Prabhupāda: No, no. Formerly they were under the impression the world is flat. And now they are saying round. So what they'll say after few years?

Yaśodā-nandana: They are not consistent. That's a fact. They're very inconsistent in their theories.

Prabhupāda: Ask them which is correct. "Probably" this is correct.

Bhakti-prema: When someone asks this question, first one would reply.

Prabhupāda: I answered it. You people say like that, so which is correct? Flat or round?

Tamāla Kṛṣṇa: They will say "What does the Bhāgavatam say?"

Prabhupāda: Huh?

Tamāla Kṛṣṇa: They may reply to us, "All right, we are rascals. So please tell us what is the fact."

Prabhupāda: That is, we are...

Tamāla Kṛṣṇa: That's what he's asking. What should be shown? Actually we're a little stumped by... I mean...

Prabhupāda: Yes, you must have proper answer as far as possible.

Tamāla Kṛṣṇa: We just read... We got a version from South India, and we've even found that there are different conceptions of what the Bhāgavata is saying. But the Purāṇas, they give some Puranic references.

Bhakti-prema: It is written the world... The earth is round and flat.

Prabhupāda: Hm?

Bhakti-prema: Earth is round and flat both, together.

Prabhupāda: Yes.

Bhakti-prema: First we should reply it is acintya. This should be the reply. "Inconceivable."

Yaśodā-nandana: If it is inconceivable, then they will say how we can conceive it?

Prabhupāda: Take the version of Bhāgavatam.

Bhakti-prema: Everything we conceived, that is wrong?

Prabhupāda: Everything you conceived, that is wrong. Yes. Therefore inconceivable.

Bhakti-prema: The Lord is inconceivable always and any (indistinct), it is inconceivable.

Prabhupāda: But we have to accept śāstra.

Bhakti-prema: This
...
https://www.youtube.com/watch?v=q5at2i6Tnvo