Investors are getting nervous about how much debt Big Tech is taking on to build AI, and it's showing up as a warning sign in the credit markets.

• Companies like Oracle, Nvidia, Alphabet, Amazon, and Meta are borrowing huge sums to build data centers and buy chips, but the revenue to pay for it hasn't shown up yet.
• The cost to insure their debt against default is hitting record highs, meaning big investors are quietly bracing for trouble.
• Oracle got its credit rating cut to just one notch above junk after saying it will spend $70 billion on data centers.
• Nvidia may back $250 billion in financing to help OpenAI build a huge data center, part of a "circular" loop where Nvidia funds the customers who then buy its chips — regulators call this a systemic risk.
• Hidden and off-balance-sheet debt tied to these firms has reportedly ballooned to around $1.65 trillion, eight times higher than four years ago.

Outlook: No default looks imminent, but if AI spending doesn't start paying off soon, the rising cost of insuring this debt points to more stress ahead — and possibly a broader market drop, since Nvidia alone helps hold up the whole market.